I understand that some words like trousers,scissors, ... are used in a plural form and to refer to a single pair we can use "a pair of". So whether sentences like "my new trousers are too long" can have two meanings (multiple pair of trousers and a pair of trousers) depends on situations?
For example, you have bought more than one pair of trousers and you want to say that all of them are too long versus when you've bought just a pair and want to say it's too long.
Yes this is true. But it is not a problem.
For example the "The apples are too sour" could have several different meanings. There could be two apples or three or even four or more. You can't tell from the words how many apples there are, so it is ambiguous. In languages which have singular-dual-plural the case where there are two apples would have to be translated differently.
In English the word "trousers" is odd, since it is grammatically plural, but can refer to a single item of clothing. So "The trousers I bought yesterday are too long" is ambiguous, There could be one pair of trousers, or two or even three or more. In languages where the word is grammatically singular the case where there is one pair would have to be translated differently.
But this case is no more ambiguous than the example with apples.
The speaker can choose to be unambiguous, by using a number:
I bought three pairs of trousers yesterday, and they were all too long.
Without further context, I'd assume that you'd bought one pair of trousers, since if you'd bought several pairs, I think you'd mention it explicitly.
There are two parts to answer this question the first of which has been over looked.
With regard to trouser
the use of trouser is quite common, even if we cannot buy a trouser.
trouser: adjective C.E.D.[ before noun ] mainly UK; belonging or relating to trousers:
"Just look at this scar," said Aaron proudly, rolling up his trouser leg.
Therefore your statement
I understand that some words like trousers, scissors, ... are used in a plural form
is correct because the item consists of two pieces, therefore we use the plural form of the subject just as we would when refer to two items of anything, like socks.
However the pivotal point of your question arises from the use of "a pair".
The item or object referred to in "the paring" being plural however a pair is singular.
Now with regard to the Trousers problem
A simple answer to this is treat it as you would normally. However the secret is not how to say it but saying it correctly. You did not buy some trousers you bought a pair of trousers. Trouser are sold as a pair like socks. Even if this is commonly referred to as just trousers.
So we have a belt and it is too long
I bought a belt but it is too long
This meets the requirements to denote a singular object.
I bought some belts but they are too long.
This meets the requirements to denote multiple (plural) objects.
Now do the same for the trousers
I bought a pair of trousers but they are too long
This meets the requirements denote a singular object (A PAIR OF TROUSERS), however after the fact we have to use the plural state for trousers.
I bought some pairs of trousers but they are too long.
Just like with the belts or any other item it would be normal to add "they are all too long" this just emphasises the point of every item being too long. It is also normal to mention the quantity but it is not a necessity. With belts or any other item it normal to use a world relating to an amount. "a few", "several" or "a lot off"
I bought some pairs of trousers but they are all too long.
This meets the requirements to denote multiple (plural) objects.
trousers; noun [ plural ] mainly UK; a piece of clothing that covers the lower part of the body from the waist to the feet, consisting of two cylinder-shaped parts, one for each leg, that are joined at the top: Ref C.E.D.
Example
I need a new pair of trousers to go with this jacket.
It is interesting we still tend to say "I bought a pair of socks", whilst it has become common place to say "I bought some new trousers" after purchasing a pair of trousers.
I bought a pair of thick woollen socks.
No doubt because the ambiguity of "I bought some socks" is obvious. Normally if you used "some socks" it would be usual to assume you bought several pairs.
