Triunghiul reflectorizant este în mașină.

Because Romanian usually puts the definite article at the end of the noun, not in front of it.

• triunghi = triangle
• triunghiul = the triangle

So -ul here is the equivalent of English the.

Triunghi is a neuter noun.

Romanian neuter nouns behave:

• like masculine in the singular
• like feminine in the plural

That is why in the singular you get:

• triunghiul reflectorizant

But in the plural you would get:

• triunghiurile reflectorizante

So the adjective changes in the plural, even though it does not change here.

Because in Romanian, descriptive adjectives usually come after the noun.

So:

• triunghi reflectorizant = reflective triangle

This is the normal order. English often puts adjectives before the noun, but Romanian usually puts them after.

In the normal Romanian pattern noun + adjective, the definite article usually appears only on the noun:

• triunghiul reflectorizant

Not:

• triunghiul reflectorizantul

The adjective still agrees with the noun in gender and number, but it normally does not take its own article in this position.

Este means is. It is the 3rd person singular of a fi = to be.

Yes, in everyday speech, many Romanians would say:

• Triunghiul reflectorizant e în mașină.

That is more conversational.
Este is a little more neutral or formal.

În mașină is the natural Romanian way to say in the car or in a car, depending on context.

Romanian often uses the bare noun after a preposition in places where English uses the.

So:

• în mașină = in the car / in a car

If you say în mașina..., it usually needs something after it, such as:

• în mașina mea = in my car
• în mașina roșie = in the red car
• în mașina din față = in the car in front

You would normally use un:

• un triunghi reflectorizant = a reflective triangle

So a full sentence could be:

• Un triunghi reflectorizant este în mașină.

However, Romanian often prefers a different structure for introducing something new, such as:

• Există un triunghi reflectorizant în mașină.
• În mașină este un triunghi reflectorizant.

În means in or inside.

It is a very common preposition:

• în casă = in the house
• în oraș = in the city
• în mașină = in the car

So in this sentence, it marks location.

Mașină is pronounced roughly like mah-SHEE-nuh.

A few helpful details:

• ș sounds like English sh
• ă sounds like a short, relaxed uh
• the stress is on the middle syllable: ma-ȘI-nă

So:

• mașină → mah-SHEE-nuh

It is roughly tree-OON-gyool, with the stress on OON.

Helpful breakdown:

• tri-
• -un- = stressed
• -ghiul = a compact ending with a hard g sound

The ngh part can be tricky. You can think of it as something like the ng in song, followed by a hard g glide.

You do not need perfect pronunciation right away; this is a word many learners find awkward at first.

Yes, but the original sentence is the most neutral and natural order:

• Triunghiul reflectorizant este în mașină.

If you change the order, you change the emphasis. For example:

• În mașină este triunghiul reflectorizant.
  This emphasizes the location.

But for a basic statement, the original order is the best one to learn first.

No. Triunghi reflectorizant is understandable and common, but for the road-safety item itself, you may also hear:

• triunghi de avertizare = warning triangle

So depending on context, both can appear.
In this sentence, reflectorizant focuses on the fact that it is reflective.