Can you please check if these sentences are grammatically correct,please? Thank you very much.

1)It’s two years since I last saw him (OR I saw him last?)
How long is it since you last saw him?
2) I last (is last placed between subject and verb?) saw him two years ago.
3) How long ago did you last see him?
I haven’t been here for a long time. (= It’s a long time since I last was here)
4) I haven’t been here (for) long. =(I arrived a short time ago; I haven been here for a short time)
I’ve been looking through all my sports equipment all morning (but since this morning?) but I haven’t found my swimming trunks yet.
5) I’ve been thinking about selling my old computer games for a long time, but I haven’t decided yet.

1)It’s two years since I last saw him (OR I saw him last?) Either one is correct; be sure to put a period at the end of the sentence.

How long is it since you last saw him?
OK

2) I last saw him two years ago. OK; and yes, "last" belongs right where you put it.

3) How long ago did you last see him?
I haven’t been here for a long time. (= It’s a long time since I last was here)
All are OK.

4) I haven’t been here long. OK

I arrived a short time ago. Don't use the second sentence here.

I’ve been looking through all my sports equipment all morning, ) but I haven’t found my swimming trunks yet.

5) I’ve been thinking about selling my old computer games for a long time, but I haven’t decided yet. OK

Sure! Let's go through each sentence and check if it is grammatically correct.

1) "It’s two years since I last saw him (OR I saw him last?)" Both variations are grammatically correct, but they have slightly different meanings. The first sentence emphasizes the duration of time since you last saw him, while the second sentence simply states that you saw him last.

To answer the question "How long is it since you last saw him?", you can say "It has been two years since I last saw him" or "I last saw him two years ago."

2) "I last (is last placed between subject and verb?) saw him two years ago." The word "last" is placed correctly in this sentence. It comes before the verb "saw" to indicate the specific timeframe of two years ago.

3) "How long ago did you last see him?" This sentence is grammatically correct. It asks about the time that has passed since you last saw him.

"I haven’t been here for a long time. (= It’s a long time since I last was here)" This sentence is also grammatically correct. It expresses that you haven't been at the current location for a significant period of time.

4) "I haven’t been here (for) long. =(I arrived a short time ago; I haven been here for a short time)" Both variations are grammatically correct and convey the same meaning. They indicate that you recently arrived at the current location.

"I’ve been looking through all my sports equipment all morning (but since this morning?) but I haven’t found my swimming trunks yet." This sentence is grammatically correct. It states that you've been searching for your swimming trunks throughout the morning, but you haven't found them yet. You could also say "since this morning" to clarify the starting point of your search.

5) "I’ve been thinking about selling my old computer games for a long time, but I haven’t decided yet." This sentence is grammatically correct. It expresses that you have been contemplating selling your old computer games for an extended period, but you haven't made a decision yet.