Certainly is used to emphasize statements. You often use certainly when you are agreeing with something that has been said or confirming that something is true.

Be Careful!
Don't confuse certainly and surely. You use surely to express disagreement or surprise.

Both British and American speakers use certainly to respond positively to a question or statement.

American speakers also use surely in this way.

Certainly is usually used to modify verbs.

If there is no auxiliary verb, you put certainly in front of the verb, unless the verb is be.

If the verb is be, certainly can go either in front of it or after it. It usually goes after it.

If there is an auxiliary verb, you usually put certainly after the auxiliary verb.

If there is more than one auxiliary verb, you usually put certainly after the first one. Certainly can also go in front of the first auxiliary verb.

If you use an auxiliary verb without a main verb, you put certainly in front of the auxiliary verb.

You can also put certainly at the beginning of a sentence.

If you think that something is true, but you are not quite sure about it, you can use almost certainly.

Be Careful!
Don't put 'nearly' in front of certainly.

You use surely for emphasis when you are objecting to something that has been said or done.

Don't use 'surely' simply to give strong emphasis to a statement. Use definitely.

In British English, you don't use 'surely' when you are agreeing with something that has been said, or confirming that something is true. Use certainly.

American speakers use both surely and certainly to agree with requests and statements.

Don't use 'surely' to say emphatically that something will happen in the future. Use definitely or certainly.

Don't use 'surely' to emphasize that something is what you would expect in particular circumstances. Use naturally.