How do you use "look forward"?

- Sometimes I have seen it followed by a gerund

- Sometimes I 've seen it followed by a present time verb.

Which one is correct? or when do you use each?

look forward to ---ing ...

to normally follows look forward.

I look forward to seeing you again.

I look forward to continuing with my studies.

I look forward to being able to speak English.

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 CalifJim's reply was promoted to an answer.
Thanks CJ. a much better way explaining.