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 Which Word? affect / effect
affect verb = 'to have an influence on sb/sth': Does television affect children's behaviour? It is not a noun.
effect noun = 'result, influence': Does television have an effect on children's behaviour?
effect verb is quite rare and formal and means' to achieve or produce'. They hope to effect a reconciliation.

although / even though / though
You can use these words to show contrast between two clauses or two sentences. Though is used more in spoken than in written English. You can use although, even though and though at the beginning of a sentence or clause that has a verb. Notice where the commas go: Although/Even though/Though everyone played well, we lost the game. We lost the game, although/even though/though everyone played well.
You cannot use even on its own at the beginning of a sentence or clause instead of although, even though or though.

answer / reply
Verbs
Answer and reply are the most common verbs used for speaking or writing as a reaction to a question, letter, ect.
Note that you answer a person, question or letter, not answer to them, but you reply to somebody or something: I'm writing to answer your questions. I'm writing to reply to your question.
Answer can be used with or without an object: I haven't answered her email yet. I knocked on the door but nobody answered. Reply is often used with the actual words spoken: 'I won't let you down, 'he replied.
You can only answer a door or a phone.
Nouns
Note the phrases in answer to and in reply to: I'm writing in answer to your letter.   

begin / start
There is not much difference in meaning between begin and start, though start is more common in spoken Enlgish: What time does the concert start/begin? She started/began working here three months ago.
Begin is often used when you are describing a series of events: The story begins on the island of Corfu. Start, but not begin, can aslo mean 'to start a journey' , 'to start something happening' or 'to start a machine working': We''ll need to start at 7:00. Who do you think started the fire? The car won't start.
You can use either an infinitive or a form with -ing after begin and start, with no difference in meaning: I didn't start worrying/to worry until she was 2 hours late.
After the forms beginning and starting, the -ing form of the verb is not normally used: It's starting/beginning to rain.   

can / may
Can and cannot (or can't) are the most common words used for asking for, giving or refusing permission: Can I borrow your calculator? You can come with us if you want to. You can't park your car there.
May (negative may not) is used as a polite and fairly formal way to ask for or give permission: May I borrow your newspaper? You may come if you wish. It is often used in official signs and rules: Visitors may use the swimming pool between 7 a.m. and 7 p.m. Students may not use the college car park. The form mayn't is hardly ever used in modern English.