1. My hobby is internet games and paper folding.

2. My hobbies are internet games and paper folding.
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Which one is grammatical?

3. My hobby is exchanging text messages on Kakao Talk on my smart phone.

4. My hobby is talking with my friends on the phone.

5. My hobby is group chatting.
6. My hobby is chatting in groups on-line.
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Are they all grammatical? Which one is ungrammatical?
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7. My hobby is using the computer.
8. My hobby is to use the computer.
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Can we use both expressions? Which one is commonly used? Do we have to use the ing-form or to infinitive form?

Q1) 2 is GRAMMATICALLY CORRECT

Q2) 5 is GRAMMATICALLY CORRECT the rest are NOT GRAMMATICALLY CORRECT
Q3) 8 is the best one. If I were phrasing ANY of these sentences I would do it differently, however 8 is the best choice.

Carpe_Diem is wrong. All except #1 are correct. #1 is wrong because you list two hobbies, not one. The verb should be the plural "are", not "is". Your hobbies ARE (two things), as in #2.

1. Both sentences are grammatical, but they have slightly different meanings.

2. Sentence 2, "My hobbies are internet games and paper folding," suggests that the person has multiple hobbies, namely internet games and paper folding.

3. Sentence 3, "My hobby is exchanging text messages on Kakao Talk on my smartphone," is grammatical and indicates that the person's specific hobby is texting on Kakao Talk using their smartphone.

4. Sentence 4, "My hobby is talking with my friends on the phone," is grammatical and communicates that the person's hobby is having conversations with their friends over the phone.

5. Sentence 5, "My hobby is group chatting," is grammatical and means that the person enjoys participating in conversations with multiple people in a group chatting context.

6. Sentence 6, "My hobby is chatting in groups online," is also grammatical and conveys the same idea as sentence 5, but in a slightly different way.

7. Both sentences 7, "My hobby is using the computer," and 8, "My hobby is to use the computer," are grammatical, but sentence 7, which uses the gerund form ("using"), is more commonly used.

8. Generally, the gerund form (using the -ing form) is more commonly used when expressing hobbies. So, sentence 7, "My hobby is using the computer," is the more commonly used and preferred form. However, the to-infinitive form ("to use") in sentence 8 is also grammatically correct and can be used in certain contexts, but it is less common in this particular context of expressing hobbies.