Trusa medicală este în mașină.

Because Romanian usually puts the definite article at the end of the noun.

• trusă = a kit
• trusa = the kit

So trusa medicală means the medical kit.

For comparison:

• o trusă medicală = a medical kit
• trusa medicală = the medical kit

In Romanian, descriptive adjectives usually come after the noun.

So the normal, neutral order is:

• trusa medicală = the medical kit

This is more natural than putting the adjective first. You can sometimes place an adjective before the noun, but that usually sounds more literary, emphatic, or stylistically marked.

Because the adjective has to agree with the noun in gender and number.

trusă is a feminine singular noun, so the adjective must also be feminine singular:

• masculine singular: medical
• feminine singular: medicală

That is why you get trusa medicală, not trusa medical.

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

Yes, in everyday Romanian, e is very common:

• Trusa medicală este în mașină.
• Trusa medicală e în mașină.

Both are correct. Este sounds a bit fuller or more careful; e is more conversational.

Also, unlike in some languages, Romanian normally keeps the verb to be in this kind of sentence.

Because în means in / inside.

So:

• în mașină = in the car / inside the car

If you used la, the meaning would change. La is more like at / to, not necessarily inside.

Compare:

• în mașină = in the car
• la mașină = at the car / by the car

Romanian and English do not use articles in exactly the same way.

In everyday Romanian, a phrase like în mașină is very natural and can easily mean in the car when the context makes it clear which car is meant.

If you want to make the noun clearly indefinite, you can say:

• într-o mașină = in a car

If you want to identify the car more specifically, you often add something:

• în mașina mea = in my car
• în mașina roșie = in the red car

So în mașină is a normal, idiomatic way to express the location.

A rough guide:

• ă = a short, relaxed vowel, similar to the a in sofa
• ș = sh
• î = a Romanian vowel with no exact English equivalent; as a beginner, you can think of it as a very central, tight vowel

Approximate pronunciation of the whole sentence:

• Trusa ≈ TROO-sa
• medicală ≈ meh-dee-KA-luh
• este ≈ ES-teh
• în ≈ a short sound somewhat like uhn, but not exactly
• mașină ≈ mah-SHEE-nuh

The most important thing at first is to clearly distinguish:

• ș from s
• ă from a
• î from i

The given order is the most neutral one:

• Trusa medicală este în mașină.

Romanian does allow some flexibility, but changing the order usually changes the focus or emphasis.

For example:

• În mașină este trusa medicală.
  This puts more emphasis on where it is.

The original version is the safest and most natural choice for a learner.

Yes, they do.

Correct spelling is:

• Trusa medicală este în mașină.

The diacritics help show both pronunciation and meaning:

• ș is not the same as s
• ă is not the same as a
• î is not the same as i

People may still understand masina or in in casual typing, but that is nonstandard. As a learner, it is best to write:

• medicală
• în
• mașină

from the start.

It is in the accusative, which is the case Romanian normally uses after many prepositions, including în.

In this sentence, though, you do not see a big change in the noun form, because for many nouns the accusative looks the same as the basic dictionary form.

So:

• mașină is the noun
• after în, it is functioning in the accusative

This is useful to know, but at an early stage the main thing to remember is simply:

• în + noun = in + noun
• în mașină = in the car / in a car, depending on context