Πρόσεχε με τη σάλτσα, γιατί θα λερώσεις πάλι το πουκάμισό σου.

Πρόσεχε is the present imperative of προσέχω, and it suggests ongoing carefulness: be careful / keep being careful.

By contrast, πρόσεξε is the aorist imperative, which often sounds more like a single warning or one-time action: watch out!

In this sentence, the speaker means something like be careful with the sauce as you handle it, so the present imperative Πρόσεχε is very natural.

Here με means with, but in a practical sense: be careful with the sauce.

With προσέχω, Greek often uses με + accusative to show the thing you should handle carefully or watch out about. So this is not a literal together with meaning. It is more like:

• be careful with the sauce
• watch it with the sauce

The full feminine accusative article is την, but the final -ν is often dropped before certain consonants. Since σάλτσα starts with σ, Greek normally uses τη σάλτσα.

So:

• την
  • vowel or certain consonants
• τη before many other consonants, including σ

That is why τη σάλτσα looks natural here.

Both are in the accusative.

• τη σάλτσα is accusative because it comes after the preposition με
• το πουκάμισό σου is accusative because it is the direct object of θα λερώσεις

So Greek is marking the sauce as the object of the preposition and the shirt as the thing that will get stained.

Here it means because.

γιατί can mean either:

• why in a question
• because in an answer or explanation

In this sentence it introduces the reason for the warning, so it means because.

It is the future.

In Modern Greek, the future is usually made with θα + verb form. So:

• θα λερώσεις = you will stain / you will get dirty

The form λερώσεις is not a standalone future ending in the way English learners might expect. It is the form used after θα to express a simple future.

This is a question of aspect, which is very important in Greek.

• θα λερώσεις presents the action as one complete event/result
• θα λερώνεις would suggest something ongoing, repeated, or habitual

Here the idea is you’ll get your shirt dirty again as a single result, so θα λερώσεις is the right choice.

Greek usually does not need a subject pronoun, because the verb ending already shows who the subject is.

Here both verbs show second person singular:

• Πρόσεχε = you, be careful
• θα λερώσεις = you will stain/get dirty

So Greek does not need εσύ unless the speaker wants emphasis, contrast, or emotion.

πάλι means again.

It shows that this is not the first time the person has done this. So the speaker is implying something like:

• you’re going to dirty your shirt again
• you’ll get your shirt messy again, like before

Its position is fairly natural here, and it modifies the whole idea of dirtying the shirt again.

This happens because σου is an enclitic. Enclitics are little unstressed words that lean on the previous word.

The noun πουκάμισο is stressed on the antepenultimate syllable. When an enclitic like σου follows it, Greek often adds an extra written accent to keep the stress pattern clear:

• το πουκάμισο
• το πουκάμισό σου

So the extra accent is not random; it reflects a real pronunciation rule.

That is the normal Greek way to express possession with weak possessive forms.

Greek usually says:

• the shirt your = το πουκάμισό σου

rather than putting the possessive before the noun the way English does.

So two important things are happening:

• σου comes after the noun
• the noun normally keeps the definite article: το

This is why το πουκάμισό σου is the normal Greek equivalent of your shirt.

σου is second person singular, so the sentence is addressing one person informally.

That matches the verb forms too:

• Πρόσεχε
• θα λερώσεις

If you were speaking to more than one person, or using polite plural, Greek would use σας and different verb forms. So yes, this sentence is clearly singular and informal.