Η τρόμπα είναι στην τσάντα.

Είναι is the present-tense form of είμαι, meaning to be. Here it means is.
So in this sentence, η τρόμπα είναι... = the pump is...

A useful extra point: είναι is also the form used for they are, so context tells you whether it is singular or plural.

Στην is the usual contracted form of σε την.

• σε = in / at / to / on
• την = the feminine accusative form of the

So:

σε την τσάντα → στην τσάντα

This is very common in Greek. In this sentence, στην means in the.

After the preposition σε, Greek uses the accusative case.

So the noun phrase the bag changes like this:

• nominative: η τσάντα
• accusative: την τσάντα

Then, because σε + την contracts, you get:

• στην τσάντα

Notice that the noun τσάντα itself looks the same here; the article is what clearly shows the case change.

No. Σε is a very flexible preposition. Depending on context, it can mean:

• in
• at
• to
• sometimes on

In this sentence, στην τσάντα is understood as in the bag.

A good approximation is TROM-ba.

A few details:

• τρ sounds like tr
• ό shows the stress is on the first syllable
• μπ in the middle of a word is usually pronounced like b, or sometimes with a slight mb sound depending on the speaker

So τρόμπα is roughly TROM-ba.

A good approximation is TSAHN-da.

The key part is τσ, which sounds like ts, as in the end of cats.

So:

• τσ = ts
• ά shows the stress is on that syllable

The accent marks show which syllable is stressed.

• τρόμπα → stress on τρό
• είναι → stress on εί
• τσάντα → stress on τσά

Stress matters in Greek, so the written accent is important.

No, Greek word order is more flexible than English word order.

Η τρόμπα είναι στην τσάντα is the most neutral, straightforward way to say it.

But Greek can also move things around for emphasis, for example:

Στην τσάντα είναι η τρόμπα
This can sound like It’s in the bag that the pump is or The pump is in the bag with extra focus on in the bag.

Yes. Μέσα στην τσάντα means inside the bag, which is a bit more explicit.

So:

• Η τρόμπα είναι στην τσάντα = normal, natural
• Η τρόμπα είναι μέσα στην τσάντα = the pump is inside the bag

Both are correct; μέσα just adds emphasis to the idea of being physically inside.