Эти джинсы слишком тесные.

In Russian, джинсы is also treated as a plural-only noun. That means it normally appears only in the plural, even when you mean one pair of jeans.

So Russian agrees with it as plural:

• эти джинсы = these jeans
• новые джинсы = new jeans
• джинсы тесные = the jeans are tight

This is similar to other clothing items like:

• брюки = trousers/pants
• шорты = shorts

Because эти is the plural form of this/these.

The word джинсы is plural, so the demonstrative has to match it:

• этот = this (masculine singular)
• эта = this (feminine singular)
• это = this (neuter singular)
• эти = these (plural)

Since джинсы is plural, Russian uses эти:

• Эти джинсы = These jeans

Because тесные is an adjective, and it must agree with джинсы in number, gender, and case.

Here, джинсы is:

• plural
• nominative case

So the adjective also takes the plural nominative form:

• singular masculine: тесный
• singular feminine: тесная
• singular neuter: тесное
• plural: тесные

That is why you get:

• Эти джинсы слишком тесные.

It is in the nominative case.

This sentence is basically structured like:

• These jeans
  • are
    • too tight

In Russian, the subject of the sentence is usually in the nominative, so:

• эти джинсы = nominative plural
• тесные also matches in nominative plural

In the present tense, Russian usually omits the verb to be.

So English:

• These jeans are too tight.

becomes Russian:

• Эти джинсы слишком тесные.

There is no separate word for are here. This is completely normal in Russian.

In other tenses, a form of to be does appear:

• Эти джинсы были слишком тесные. = These jeans were too tight.
• Эти джинсы будут слишком тесные. = These jeans will be too tight.

Слишком means too in the sense of more than is acceptable or comfortable.

So:

• слишком тесные = too tight

It is not just very. Compare:

• очень тесные = very tight
• слишком тесные = too tight

So слишком suggests a problem.

Because слишком modifies the adjective тесные, just like English too modifies tight in too tight.

So the normal order is:

• слишком тесные = too tight

This placement is the most natural and standard one.

Yes, Russian word order is fairly flexible, but the neutral, most natural order here is:

• Эти джинсы слишком тесные.

You may also see reordering for emphasis, for example:

• Слишком тесные эти джинсы.
  This sounds more marked, like These jeans are just too tight.

For a learner, it is best to stick with:

• Эти джинсы слишком тесные.

Not exactly.

Тесный means tight or too close-fitting. It describes how the clothing fits on the body.

Compare:

• тесные джинсы = tight jeans
• маленькие джинсы = small jeans

Sometimes clothing can be маленькое because the size is too small, and as a result it feels тесное. But the meanings are still different:

• маленький = small in size
• тесный = tight in fit

Sometimes, but the meaning changes.

• тесные = tight, too tight on the body
• узкие = narrow

For jeans:

• тесные джинсы usually means they fit too tightly
• узкие джинсы can describe the style, like narrow/slim-cut jeans

So if you mean the jeans are uncomfortable because they are too tight, тесные is the better word.

A helpful stress pattern is:

• Э́ти джи́нсы сли́шком те́сные.

Approximate pronunciation:

• EH-ti JHEEN-sy SLEESH-kam TYES-ny-ye

A few notes:

• дж sounds like the j in jam
• жи sounds roughly like zhee
• ы in джинсы is a Russian vowel with no exact English equivalent
• тесные has a consonant cluster, so say it carefully: TEHS-ny-ye

With эти, it clearly means these jeans.

If you remove эти, then:

• Джинсы слишком тесные.

can mean The jeans are too tight or Jeans are too tight, depending on context.

So эти points to a specific pair and makes the meaning more like these.

Yes. That is another very common pattern.

• Эти джинсы слишком тесные. = These jeans are too tight.
• Эти джинсы мне тесны. = These jeans are tight on me / too tight for me.

In мне тесны, the short-form adjective тесны is used in a more predicative way, often with the person affected:

• мне тесно = it feels cramped/tight to me
• ботинки мне тесны = the shoes are tight on me

For learners, Эти джинсы слишком тесные is easier and very natural, but мне тесны is also useful to know.