Το γκρι παντελόνι είναι χαλαρό στη μέση, αλλά το μπλε είναι πολύ σφιχτό.

In Greek, παντελόνι is normally a singular neuter noun meaning one pair of trousers/pants. So Greek says:

Το παντελόνι είναι... = The pants are...

That means the verb is singular too: είναι here is the same form for singular and plural, but the noun itself is grammatically singular.

Because they agree with παντελόνι, which is neuter singular.

• το παντελόνι
• χαλαρό
• σφιχτό

Greek adjectives usually match the noun in gender, number, and case. Since παντελόνι is neuter singular, the adjectives are neuter singular too.

Usually no. γκρι and μπλε are commonly used as indeclinable color adjectives, so they normally stay the same:

• το γκρι παντελόνι
• η γκρι φούστα
• τα γκρι παπούτσια

and

• το μπλε παντελόνι
• η μπλε μπλούζα
• τα μπλε παπούτσια

So unlike χαλαρός / χαλαρή / χαλαρό, these color words usually do not change their endings.

Because Greek, like English, often omits a noun when it is already clear from context.

So:

αλλά το μπλε είναι πολύ σφιχτό

means:

but the blue one is very tight

The full version would be:

αλλά το μπλε παντελόνι είναι πολύ σφιχτό

but repeating παντελόνι is unnecessary here.

Because in Greek, an article + adjective can act like a noun phrase:

• το μπλε = the blue one
• το κόκκινο = the red one
• η μεγάλη = the big one / the big one (feminine)

Here το shows that the missing noun is something neuter singular, namely παντελόνι.

στη μέση means at the waist or around the waist in this sentence.

στη is the contracted form of:

σε + τη = στη

So:

• σε τη μέση becomes στη μέση

This is very common in Modern Greek.

No. μέση can mean:

• waist
• middle
• center

The meaning depends on context.

Here, because we are talking about clothing fit, στη μέση clearly means at the waist.

Here πολύ means very. It works as an adverb modifying the adjective σφιχτό:

• πολύ σφιχτό = very tight

So although πολύ can also mean much / a lot, in this sentence it means very.

In clothing context:

• χαλαρό = loose
• σφιχτό = tight

So:

• είναι χαλαρό στη μέση = it is loose at the waist
• είναι πολύ σφιχτό = it is very tight

These are natural opposites when describing how clothes fit.

Yes, but το γκρι παντελόνι is the most straightforward, neutral order here.

Greek can also place descriptive elements after the noun in some contexts, but for a simple sentence like this, article + color + noun is the most natural pattern for learners to recognize and use first:

• το γκρι παντελόνι
• το μπλε πουκάμισο

So this word order is a good default.

αλλά means but and introduces a contrast.

The sentence is comparing two items:

• the grey trousers are loose at the waist
• the blue one is very tight

So αλλά is the natural connector because the second part contrasts with the first.

Greek usually does not need a separate word for one in this kind of sentence. It simply uses the article + adjective:

• το μπλε = the blue one

This is very common and natural in Greek. English often needs one, but Greek often does not.