The reason clean energy is hard to come by is that we've spent the past 150 years depending on oil, and it's quite difficult to make the switch. Here "difficult to make the switch" is related to

Substitutions

অনেকগুলো শব্দকে এক শব্দে পরিণত করাকে ইংরেজিতে বলে One word substitution বা এক কথায় প্রকাশ; যেমন — A person who writes dictionaries = Lexicographer অন্যদিকে ইংরেজিতে কিছু কিছু শব্দগুচ্ছ আছে যেগুলো বিশেষ অর্থ প্রদান করে। তাদেরকে Fixed expressions বলা হয়। পরীক্ষায় অনেক সময় বিশেষ বিশেষ শব্দ যা পরিভাষার বিশ্লেষণ বা সংজ্ঞা জানতে চাওয়া হয় অথবা সংজ্ঞা দেয়া থাকে যা দেখে বিশেষ শব্দটি বের করতে হয়।

One word substitution : এক কথায় প্রকাশ বিভিন্ন ক্ষেত্র/বিষয় থেকে আসতে পারে। এখানে গুরুত্বপূর্ণ কিছু ক্ষেত্র/বিষয় নিয়ে আলোচনা করা হলো :

Study of religion-Theology

Study of heredity -- Genetics (বংশগতির বিজ্ঞান)

Study of coins — Numismatics (মুদ্রা ও পদকসংক্রান্ত বিদ্যা)

Study of birds— Ornithology (পক্ষীবিজ্ঞান)

Study of human development- Anthropology (নৃ-বিজ্ঞান, मরবিজ্ঞান)

 Study of science of insects -Entomology (কাটতত্ত্ব)

Study of problems of old age -Gerontology (বার্ধক্য প্রক্রিয়া বিষয়ক বিজ্ঞান)

Study of relation between organism and environment (প্রাণী ও পরিবেশের সম্পর্ক সংক্রান্ত) -Ecology

Study of flying aeroplanes – Aviation (বিমান চালনা) 

Study of earth and rocks - Geology (ভূতত্ত্ববিদ্যা)

Study of living things-Biology

Study of celestial bodies (মহাজাগতিক বন্ধু) - Astronomy

Study of the influence of planets and stars (মানুষের ওপর তারকারাজির প্রভাব) — Astrology (জ্যোতিষ শাস্ত্র)

Scientific study of bodily diseases - Pathology (রোগবিজ্ঞান)

Study of Languages - Philology (বিজ্ঞান) 

Study of bees - Apiology

Study of heart / heart diseases -Cardiology (বিজ্ঞান)

Study of trees-Dendrology 

Study of skin/skin diseases-Dermatology

Study of wine/wine making-Enology/Oenology

Study of dogs-Cynology

Study of cats-Felinology 

Study of handwriting— Graphology 

Study of medicine for women-Gynecology/Gynaecology

Study of liver-Hepatology

Study of horses-Hippology 

Study of water -Hydrology (জলবিজ্ঞান; জলানুসন্ধানবিদ্যা)

Study of weather-Meteorology

 Study of kidneys and their diseases - Nephrology

Study of nerves-Neurology

Study of cancer-Oncology

Study of eggs-Oology

Study of eyes-Ophthalmology

Study of mountains and their mapping-Orology 

Study of bones-Osteology

Study of rocks-Petrology

Study of earthquakes-Seismology

 Study of snakes -Serpentology

Study of mouth and its diseases-Stomatology 

Study of poisons -Toxicology

Study of flags-Vexillology

Study of animals-Zoology

 Study of medicine for children and their diseases - Paediatrics/Pediatrics

Study of broken bones-Orthopaedics

Study of handwriting-Graphology

Art of good handwriting-Calligraphy

legible or bad handwriting-Cacography 

A written message from far off place - Telegraph

An expert in writing by hand-Chirographer

A person who write dictionaries-Lexicographer 

Written by light-Photograph

An expert in writing (drawing) maps-Cartographer 

Writing of one's life story-Biography

Writing of one's own life story-Autobiography

A process of printing from a plate-Lithography

Self-rule or control-Autonomy

A self moving vehicle- Automobile 

A machine that functions by itself - Automatic

A signature of oneself -Autograph

One with unlimited power-Autocrat 

Love for mankind-Philanthropy

One who loves books as collection-Bibliophile 

One who loves women--Philogyny

One who loves men-Androphile

One who loves wisdom-Philosopher

One who loves and admires the British-Anglophile

One who is all powerful-Omnipotent,

Almighty One who is all knowing-Omniscient 

One who is present all over-Omnipresent

One who eats all kinds of food-Omnivorous

 A book containing all the published works of an author (একজন লেখকের সব লেখার সংকলন) – Omnibus

Theo= relating to God Study of religion-Theology

One who believes in God-Theist 

One who believes in one God-Monotheist

One who believes in many Gods-Polytheist

One who believes the presence of God in everything in the universe -Pantheist

Religious madness-Theomania 

Practising only one marriage (having only one wife/)-Monogamy

Practising two marriages- Bigamy

Practising several marriages (having many wives) - Polygamy

One who hates marriage-Misogamy

Having several husbands -Palyandry 

People who have both male and female sexual organs-Gynandry (ad), gynandrous)

 People having both male and female characteristics (নারী-পুরুষ উভয় বৈশিষ্ট্য যার মধ্যে)-Androgynous

The study of human development-Anthropology

One who loves mankind-Philanthropist

One who hates humans-Misanthrope

One who hates men-Misandrist

One who hates women-Misogynist 

One who hates marriage- Misogamist

One who loves his fatherland (country)- Patriot 

An inheritance from father (পিতা থেকে পাওয়া উত্তরাধিকার) – Patrimony

The state of being father - Paternity

The killing of a father - Patricide 

A system ruled by father/men-Patriarch

The rule or control of mother-Matriarchy

The killing of a mother - Matricide 

The school or college one attends - Alma Mater

The killing of one's own brother-Fratricide

The killing of one's sister-Sororicide

The killing of one's wife- Uxoricide 

The killing of one's husband-Mariticide

The killing of parents-Parricide

The killing of a whole race-Genocide 

The killing of a new born child - Infanticide

The killing of a king-Regicide 

The killing of a human being-Homicide

The killing of oneself-Suicide

To walk aimlessly-Amble

A baby carriage-Perambulator

Carriage  for sick people-Ambulance

One who walks in sleep-Somnambulist

One who walks at night-Noctambulist

One who walks on ropes-Funambulist

 Living on flesh-Carnivorous

Living on grass -Herbivorous

Living on fish (শুধু মাছ খেয়ে বেঁচে থাকে যে প্রাণী)- Pacivorous

Living on all -Omnivorous

Living on fruits-Frugivorous

Person / Personal Quality (ব্যক্তি বা ব্যক্তিগত গুণাবলি)

A person leaving his native country to settle in another - Emigrant 

A person coming to a foreign land to settle there-Immigrant 

A person looking only to the bright side of things-Optimist

A person looking only to the dark side of things-Pessimist

 A person free from national prejudices, looking at the world as his own-Cosmopolitan

A person giving up his faith-Apostate, Renegade

A person who knows many languages-Linguist

A person speaking many languages-Polyglot

A person who always doubts-Scepti

A person living at the same time with another-Contemporary

 A person who speaks for others--Spokesman

A persons working in the same place-Colleague

A person living in a foreign country-Alien; foreigner

A person unable to pay debts-Bankrupt, insolvent

A person unable to write-Illiterate

 A person devoid of knowledge-Ignorant

A person who eats only vegetable-Vegetarian

A A person who eats human flesh-Cannibal 

A person who eats excessively-Glutton

A person who treats foot diseases - Chiropodist 

A doctor who treats old people's diseases - Geriatrician

A doctor who treats pregnants and helps child birth - Obstetrician

A doctor who treats eye diseases-Ophthalmologist

A person who examines eyes and sells glasses - Optician

A person who has studied on drugs and their composition-Pharmacologist.

A doctor specialized in child care and child diseases-Paediatrician/pediatrician

A doctor who treats mental illnesses-Psychiatrist

A person/scientist who studies human mind-Psychologist

A person who foretells things by the stars-Astrologer

A person who studies heavenly bodies (মহাজাগতিক বস্তু)— Astronomer (জ্যোতির্বিদ

A person who studies ancient things and relics -Archaeologist

A person who mends shoes-Cobbler

 A person who carves on stones-Sculptor 

A person who sells medicine - Druggist

 A person who reads the palm (হস্তরেখা) - Palmist (হস্তরেখা বিশারদ)

A person who collects stamps-Philatelist

A person who deals in cattle-Drove

A person who sells flowers-Florist

A person who studies earth and rocks-Geologist

 A person who travels for selling things-Hawker 

A person/company that sells a particular product-Vendor

A person who rides the horses in races-Jockey

A person who mends waterpipes-Plumber

A person who makes earthen pots (মাটির পাত্র) – Potter (কুমার)

A person who carries burden for hire - Porter

A person who writes shorthand-Stenographer

A person who converts raw hide (কাঁচা চামড়া) into leather – Tanner (চর্মসংস্কারক)

A person who lends money at a very high rate of interest- Usurer 

A person who deals in wine-Vintner

A person in charge of a museum-Curator

A person banished from his own country-Exile

A person who spends too much-Extravagant

A person died for a noble cause-Martyr

A person who joumeys to a holy place-Pilgrim

A person who foretells events-Prophet

A person who was before another - Predecessor

A person who comes after another-Successor

A person who takes shelter /refuge in a foreign country-Refugee 

A messenger who sends message in great haste- -courier (দ্রুতবার্তাবাহক)

 Place /dwelling place

A place where dogs are kept-Kennel

A place where bees are kept-Apiary 

A place where birds are kept -Aviary

A dwelling place for wild beast -Den

A place where fishes are kept - Aquarium

The house of a gypsy -Caravan

A place for production of bread - Bakery

The underground dwelling place of an animal-Burrow 

A place for production and treatment of milk-Dairy

A place for keeping cars- -Garage A place for keeping aeroplanes - Hangar

The house of an Eskimo-Igloo

A place for discharge from the bowels -Lavatory/restroom 

A place where money is coined-Mint

A house/shelter for a confined cow-Pen

A shelter/shed for cows- -Cowshed