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When do I use gerund and when do I use infinitive. Please help.

" Swimming/To swim is a good form of exercise."

Thanks.
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In this case, use the gerund.

Swimming is a good form of exercise.

Infinitives are usually used after another conjugated verb:

I am going to swim. Do you want to swim?
Comments  
Sometimes I see that to-infinitive is a subject.

For example: To teach is to learn.

Please help. Thanks.