1) I don't know what the actual names for these things are, but I like to call them 'numeric nouns' and 'fluid nouns.' "A large number" describes a quantity of 'numeric nouns,' but a "a large AMOUNT" describes a quantity of 'fluid nouns,' which is what you should've used to describe the oxygen (The earth is the only planet with a large amount of oxygen in its atmophere). I can have one dog, or two dogs, or three dogs, but I can't have one water or two waters or three waters, but I can have SOME water. That's because dogs are 'numeric nouns' and water is a 'fluid noun.' I don't know if your native language has these differences, but if not, you simply must memorize which nouns are 'numeric' and which are 'fluid.'

1. Earth is the only planet with a large amount of oxygen in its atmosphere. 2. Venus moves/orbits closer to Earth than any other planet. 3. Robert Frost was not well-known as a poet until he reached his forties (age). OR - until the forties (as in the 1940s). 4. The amounts of oxygen and nitrogen in the air almost always remain stable, but the amount of water vapour varies considerably.

4) The first part you said correctly (The amounts of oxygen and nitrogen in the air almost always remain stable...), but the second part you said wrong (...but the amount of water vapor vary considerably). That's because nouns and verbs have to agree. If a noun is plural, its verb must be singular, and if a noun is singular, its verb must be plural. In the first part, you said "The amounts" (plural noun), and its verb was "remain" (singular verb). But in the second part, you said "The amount" (singular noun), and verb was "vary" (singular verb). Nouns and verbs are like two magnets, each with a positive and negative charge. Two similar side will repel, but two opposite sides will connect. Two correct the second clause, you'd need to switch one of them. You could've changed "The amount" to "the amounts," or you could've changed "vary" to "varies" (Y at the end of a word changes to IE if you make it plural).

3) I'm not entirely sure what you were trying to say in this one. "the forties" is used to describe the decade (1940s), but "HIS forties" would be used to describe the point in his life when he was in his 40s of years old. If you were trying to describe the decade, then you must omit "he reached." You should've said "Robert Frost was not well-known until the forties." If you were trying to describe his age, you simply say "his forties," rather than "the forties."

2) In this sentence you confused IS and DOES. Forms of is (is, isn't, are, aren't) are used to describe an object's characteristics or identity. You could say "Venus is approaching the earth more closely than any other planet is," because the phrase "is approaching" could be considered a characteristic. Forms of does (do, don't, does doesn't) are used to describe actions. "Venus approaches" describes an action Venus does. In this sentence, you would say "Venus approaches the earth more closely than any other planet does."