Welcome to the forum,
gerund means the ING form of the verb like: going/ helping/ walking etc.
Infinitive means the base form of the verb. We have infinitive with to like: to go, to walk etc and also infinitive without to like: go, walk, help.
There are different cases that we make use of them in the sentences.
1. You can use a gerund as a noun in the sentence.for example: I like apple. I like going out.Studying English is so nice.
2. There are some verbs in English that are followed either by a gerund or an infinitive. To find the long list of those verbs you can refer to a grammar reference book.
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