High school English grammar

Now I will answer your two questions respectively:

I gave him a globe and he studied it for a long time. Why use which instead of and in the blank?

First of all, if you fill in and, the second half of the sentence will become and he has studied it for a long time. (He studied it for a long time), which is linked with the first half sentence (I gave him a globe), and the logic is not smooth. It's like "learning the globe". Besides, in the second half of the sentence, if study is called "research", then it lacks an object and its grammar is not correct, so it can only be called "learning" because "learning" can be an intransitive verb. It's unreasonable to learn the globe in any way, only "learn the globe", so the space is the relative word of the attributive clause, that is, the non-restrictive attributive clause.

I suggest, I suggest, I suggest/suggest.

"I suggest" is the simple present tense and "I am suggesting" is the past continuous tense. The difference between them lies in the tense.

"I suggest" and "I suggest", the former does not exist, there is no such statement, the latter is the past tense.