I started to learn English, but it was too difficult.
I know the sentence is correct. But can I use learning instead of to learn. If not, why?
Thanks.
Yes, you can.
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See my post about "begin to rain" and "start to rain" versus "begin raining." The usage pattern is quite different. If both forms (gerund, infinitive) are allowed, each verb has preferential usage patterns in practice. Google ngrams is very useful for you to see these in the past and present.