Ma még dönthetsz, mert holnap van a határidő.

Here még means still / yet.

So Ma még dönthetsz does not just mean You can decide today. It suggests:

• the chance is still open
• but probably not for much longer

A natural English sense is:

• You can still decide today
• You still have today to decide

Because dönthetsz includes the suffix -het, which expresses possibility or permission.

• döntesz = you decide
• dönthetsz = you can decide / you may decide

It breaks down like this:

• dönt- = decide
• -het- = can / may
• -sz = you singular

So the sentence is specifically saying that the listener has the option to decide today.

Hungarian usually does not use the subject pronoun unless it is needed for emphasis or contrast.

The ending -sz in dönthetsz already tells you the subject is you singular.

So:

• dönthetsz = you can decide

You could say Te még ma dönthetsz, but that would add emphasis, something like:

• You can still decide today

Hungarian often uses the present tense for the future when the time is already clear from a word like holnap.

So:

• holnap van a határidő = the deadline is tomorrow

This works much like English:

• The deadline is tomorrow
• not necessarily The deadline will be tomorrow

The future meaning is understood from holnap.

That is true only in certain cases.

In 3rd person present, Hungarian often omits van when the predicate is a noun or adjective:

• Ő tanár. = He is a teacher.
• A ház nagy. = The house is big.

But with time, place, or similar adverbial-type predicates, van is usually kept:

• A könyv az asztalon van. = The book is on the table.
• Holnap van a határidő. = The deadline is tomorrow.

So here van is necessary.

Because van is the normal choice when you are simply stating when something falls:

• A határidő holnap van. = The deadline is tomorrow.

Using lesz can sound more like:

• it will be tomorrow
• the date is being set or changed
• or you are speaking from a point before that arrangement is fixed

In many everyday situations, van is the more natural form for a scheduled or known time.

Because it refers to a specific deadline that both speaker and listener can identify from the context.

• a határidő = the deadline

This is very natural in Hungarian. The sentence is not talking about deadlines in general, but about the particular deadline relevant to the situation.

Yes, határidő is one word. It is a compound noun:

• határ = border, limit
• idő = time

So its literal sense is something like time limit, which is why it means deadline.

Hungarian forms many everyday words this way, by combining smaller words into compounds.

Yes. Hungarian word order is flexible, and changing it often changes the focus or emphasis.

The given sentence:

• Ma még dönthetsz, mert holnap van a határidő.

is very natural and puts the time frame up front: today, still.

Other orders are possible, for example:

• Még ma dönthetsz
• Dönthetsz még ma

These are not totally different in meaning, but they can sound slightly different in emphasis. Hungarian word order is often more about what is highlighted than about strict grammar rules.

Mert is the normal Hungarian word for because.

So:

• mert holnap van a határidő = because the deadline is tomorrow

It introduces the reason for the first part of the sentence.

Hungarian also has other words that can express cause, such as mivel, but mert is the most straightforward and common choice here.

Hungarian can leave the object unstated when it is already clear from context.

So dönthetsz by itself can mean:

• you can decide
• you can make your decision

The thing being decided is understood from the situation.

This also helps explain why the verb is in the indefinite form. There is no stated definite object in the sentence. If there were a definite object, the verb form could change, for example:

• Ezt ma még eldöntheted. = You can still decide this today.