This is not such a tricky part of speech, compared, for example, to nouns or verbs. But there are a few mistakes that persecute students (and teachers) right up to the highest levels of language knowledge.
Rule №1. 40
The number 40 is written as forty. That is right, forty, not fourty, as logic suggests. Why? Why is there a letter u in the words four, fourth, fourteen, and there is no such a one in the word forty? It is unlikely that this question will be answered more accurately than "it happens". After all, until the 17th century, the number 40 was written exactly as fourty, and then the letter u was lost somewhere. Apparently, no one was worried about it, and forty became a grammatical norm.
Rule №2. Hyphen
One more thing that is easier to remember than to understand. You need a hyphen in numbers from 21 to 99 (except for multiple of ten, that is, for 30, 40, 50, etc.).
It is about thirty-two people, do not you think that twenty servings will not be enough?
How many times have I told you this, thirty-three or thirty-four? – We did not reach thirty.
For the sake of justice, we note that not all native speakers follow this rule.
Rule №3. The Mysterious History of Thousands and Hundreds
What about the next logical question? When we talk about a few thousand or hundred, do we use a plural number? The obvious question, but are you ready to answer it right now?
This rule will also have to be remembered. In numbers where there is "a certain number of thousands" (or millions, billions) these same thousands and billions are in the singular: two hundred or three thousand (and also million, billion).
Three thousand people are invited to a banquet.
In my opinion, we are talking about different numbers. Initially, it was planned to use nine million dollars, is not it?
The exceptions are the phrase "hundreds of", "thousands of," "millions of", etc., and all the other cases when we are talking about hundreds or thousands without naming a specific number.
We spent hundreds of hours to show them how to behave properly in this area.
It is about millions of rare animals that are endangered.
Rule №4. Your Favorite Percent
People fall into two categories. The first understand well percent and easily deal with it, while the second is going crazy from one realization that they will have to work with percent.
There is often confusion on this topic. You must remember that the percentage is always percent (or per cent, if you communicate with a Briton), but never percents.
50 percent of the work is completed.
I own 75 percent of the company's shares, it is pointless for you to tell me anything.
Rule №5. Numeral as Part of a Compound Adjective
And the last moment, perhaps the most difficult. Is it always necessary to put a noun in the plural after the numerals? It turns out that not always – let us understand.
The numeral in English can play 2 different roles in the sentence. It can mean the amount of something, for example, nine horses, three guitars. The second option is that it can be part of a compound adjective, for example, a three-hour session. If it still sounds confusing, then try the next trick. If the numeral answers the question "how much?" – this is the number. How much – three pieces of bread, five shirts – this is the amount. And if it answers the question "what?" – this is a compound adjective: five-star, three-yard.
The difficult part is over on this, and the rule itself is very simple. The plural is needed with quantity ("how much?"):
Today, one will bring us seven new glasses, because all the old ones were broken.
Three dogs live in their apartment.
But with the compound adjective ("what?"), on the contrary, it is not necessary. And one more thing. Since this is the compound adjective, it is written with a hyphen:
Three-meter springboard does not pose a particular danger.
It was a four-hour performance of questionable content, so I cannot say that I was satisfied.
Be careful with these rules and you will not have any grammatical problems.