[masaegu]

First, I would like to say that you have been learning well. There are several sentences that I am going to "correct" but please know that these are by my native standards. Books and courses abroad DO teach less than natural-sounding Japanese even when taught by a Japanese speaker. What you have may well get you higher scores than how native speakers would say these.

18. A strong "translated" flavor I sense. Almost all Japanese-speakers would say:

うちは６人かぞくです。

but if what you have is what they teach you, I have nothing to say though I assure you that it sounds very unnatural.

19. に　＞＞　には

５千人　＞＞　５千人の人

But the more natural way of saying this would be:

この町には５千人の人が住（す）んでいます。　住む = to live, reside

Or if I may use the word "population" as a noun:

この町の人口（じんこう）は５千人です。

20. Grammatical but unnatural.

４０人　＞＞　４０人の人

By the way, unless we are writing vertically, we rarely use kanji numbers, which is what you have seen above.

22. Grammatical but ambiguous. Your sentence CAN sound like it is Tanaka's profession.

Add いま and/or ～～ところです if you have learned the latter = ("to be in the middle of doing ~~").

田中さんはいまパーティーでうたっています。
田中さんはいまパーティーでうたっているところです。

23. Sounds very foreign to me.

あした　＞＞　あしたは = as for tomorrow

25. Grammatical but poor word choice.

先生 ＞＞　教師（きょうし）　ＯＲ　教員（きょういん）

The word 先生 is over-used by Japanese-learners. It is more an honorific than a profession.

When talking to an adult speaker, use one of the two words given above.
When talking to a kid, you may use 先生.

26. を　＞＞　が

Excellent verb tense choice!! It is the native speaker's choice! (for #27 and #28 as well)
Many Japanese-learners would have used the plain past tense there.
___________

There are more but I need to go now. Will do the rest if no one else has by then.