قبل ما تشتري القميص، جربه.

It means before when followed by a whole clause.

So:

• قبل ما تشتري القميص = before you buy the shirt

In Levantine, قبل ما is a very common everyday way to say before [someone] does something.

A rough comparison:

• Levantine: قبل ما
• MSA: قبل أن

No. Here ما is not negation.

It is just part of the common Levantine structure قبل ما = before.

So this does not mean:

• before you do not buy the shirt

It simply means:

• before you buy the shirt

This is a very common thing for English speakers to ask, because ما often does mean not in other sentences.

Because after expressions like قبل ما, Levantine usually uses the bare imperfect form, without بـ.

So you get:

• قبل ما تشتري = before you buy

rather than:

• قبل ما بتشتري

The version without بـ sounds more natural in this kind of time clause.

A useful rule of thumb:

• بتشتري often sounds like regular present/habitual you buy / you are buying
• تشتري often appears after words like before, if, when, want to, etc.

It is built into the verb.

Arabic often does not need a separate subject pronoun when the verb already shows who is doing the action.

Here:

• تشتري = you buy
• جرّب = try!

So there is no need to add إنت unless you want emphasis.

For example:

• قبل ما إنت تشتري القميص، جرّبه
  would sound more emphatic, like before you buy the shirt, try it

But normally, the sentence works perfectly without إنت.

Because -ه is the attached object pronoun meaning it (or him, for masculine nouns/people).

So:

• جرّب = try!
• جرّبه = try it!

It refers back to القميص.

Since قميص is a masculine noun, the pronoun is -ه.

This attached-pronoun pattern is very common in Arabic:

• شوفه = see it / look at it
• خده = take it
• افتحه = open it

Literally, it means try it.

But with clothing, it naturally means try it on.

So in this sentence, an English speaker would usually translate it as:

• Before you buy the shirt, try it on.

Arabic does not always need a separate word matching English on in this context. The clothing context makes the meaning clear.

Often something close to jarrbo.

That may look surprising, but it is normal in Levantine for short vowels to get reduced or dropped in fast speech.

So although the spelling is:

• جرّبه

you may hear it pronounced roughly like:

• jarrbo

Some speakers may pronounce it a little more fully, but jarrbo is a very natural Levantine-style pronunciation.

That depends on the dialect, especially the letter ق.

Two common possibilities are:

• il-qamiiS
• l-'amiiS / il-'amiiS

In many urban Levantine accents, ق becomes a glottal stop (like a quick break in the throat), so قميص may sound more like 'amiiS.

In other accents, speakers keep ق as q.

So both of these can fit Levantine pronunciation, depending on region.

Because the sentence is talking about a specific shirt — the shirt you are considering buying.

So:

• القميص = the shirt

That sounds natural here, because the speaker is probably referring to a particular shirt in front of the listener.

If the context were more general, Arabic might use an indefinite noun, but in this sentence the definite form is completely natural.

It is addressed to one person.

A big clue is the imperative:

• جرّبه = try it! (singular)

If you were talking to a group, the verb would change. For example:

• قبل ما تشتروا القميص، جرّبوه
  = Before you all buy the shirt, try it on.

So yes — the original sentence is singular.

Yes, absolutely.

That means:

• Try it before you buy it.

This version is also very natural. The difference is mostly in emphasis and structure:

• قبل ما تشتري القميص، جرّبه
  starts with the before clause
• جرّبه قبل ما تشتريه
  starts with the command try it

Also, in the second version, تشتريه means buy it, so the noun القميص does not need to be repeated.

Both patterns are common and useful.