1. He likes helping poor people.

2. It is good that he likes helping poor people.
3. I think it good that he likes helping poor people.
3-1. I think it good for him to help poor people.
3-2. I think it good to help poor people.
(Are the sentences all grammatical? I think as #1 and #2 are grammatical, the other sentences can be grammatical. Is that right?)

4. I believe it good that he likes helping poor people.
5. I feel it good that he likes helping poor people.
6. I consider it good that he likes helping poor people.
7. I find it good that he likes helping poor people.
8. I suppose it good that he likes helping poor people.
(I substituted other verbs for good. Are all the sentences grammatical?)

1, 2, 6, and 7 are fine.

If you change "it" to "it's" in the rest, those sentences will be fine, too.

Let's take a look at the grammaticality of each sentence:

1. He likes helping poor people. - This sentence is grammatically correct.
2. It is good that he likes helping poor people. - This sentence is also grammatically correct.
3. I think it good that he likes helping poor people. - This sentence is grammatically correct but sounds a bit formal or old-fashioned.
3-1. I think it good for him to help poor people. - This sentence is grammatically correct and is a slightly different way of expressing the same idea as sentence 3.
3-2. I think it good to help poor people. - This sentence is also grammatically correct and implies a general belief in the goodness of helping poor people.

4. I believe it good that he likes helping poor people. - This sentence is grammatically correct, but it sounds a bit formal or old-fashioned like sentence 3.
5. I feel it good that he likes helping poor people. - This sentence is grammatically correct, but it sounds a little unusual. Normally, we would use "think" or "believe" instead of "feel" in this context.
6. I consider it good that he likes helping poor people. - This sentence is grammatically correct and uses a different verb to express the same idea. This sentence is also formal-sounding.
7. I find it good that he likes helping poor people. - This sentence is grammatically correct and expresses a positive opinion about him helping poor people.
8. I suppose it good that he likes helping poor people. - This sentence is grammatically correct, but it sounds more uncertain or speculative compared to the other sentences.

Overall, all the sentences are grammatically correct but differ slightly in terms of formality and style.