word choice

I admit to doing something.

I admit to do something.

Both the sentences are correct. But I think the first one is used more often than the second one. Actually the second one is very rare, and old fashioned.

As others have already mentioned, it all depends on the whether the verb is followed by gerund or infinitive.

Example -

I look forward to playing on our home ground.

This is the correct sentence, but using to play here, instead to playing is considered wrong or incorrect.

But what if the verb is neutral, I mean where the verb can take both the form? Then, how to decide when to use to + verb and when to use to + verb + ing?

There is a simple rule -

Write the sentence till you reach to, and then omit the rest part for the time being. Replace the rest part with a noun. If this make sense and seems correct, you can use to + verb + ing form, otherwise stick to infinitive form to + verb

Example -

Consider the sentence -

He loves to play football.

He loves to playing football.

Following the above rule, write till to

He loves to Football.

It doesn't convey the meaning properly. So we can't use to + verb + ing form. And hence only sentence #1 is correct.

Now consider this -

Following the rule, rewrite the sentence till to

He reached closer to THE DEADLINE (A Noun). This makes sense, and so we can use to + verb + ing form, as well as the base infinitive form to + verb* form.

Does this answer make sense?