לפני שאת חותמת על המסמך, תמלאי את הטופס לאט.

No. These are two different words that are spelled the same way without vowel marks.

• In שאת, the את means you (singular feminine). It is pronounced at.
• In את הטופס, the את is the direct object marker. It is pronounced et and has no English equivalent.

So:

• לפני שאת... = before you...
• את הטופס = marks the form as the direct object

Context tells you which את you are looking at.

Several words show that clearly:

• את = you singular feminine
• חותמת = present tense, feminine singular
• תמלאי = future tense, second person feminine singular

So the sentence is addressed to one woman/girl.

If you were speaking to a singular male, it would be:

לפני שאתה חותם על המסמך, תמלא את הטופס לאט.

In Modern Hebrew, the future tense is very often used to give commands, instructions, or directions.

So תמלאי is literally a future form, but here it functions like:

• fill out
• you should fill out

This is very normal Hebrew.

There is also a true imperative form, מלאי, but the future form תמלאי is extremely common in everyday language.

This is a common Hebrew pattern.

After time words like לפני ש־ (before), Hebrew often uses a present-tense form in a clause that refers to an action that has not happened yet from the speaker's point of view.

So לפני שאת חותמת is normal Hebrew for before you sign.

You can also say:

לפני שתחתמי על המסמך...

That version is also correct and sounds a bit more explicitly future. Both are possible.

It is made of two parts:

• ש־ = that / when / which in many subordinate clauses
• את = you (singular feminine)

So:

• לפני = before
• שאת = that you / when you
• לפני שאת חותמת = literally something like before that you sign, but naturally before you sign

This is a very common structure in Hebrew subordinate clauses.

Because the two verbs behave differently.

• לחתום על = to sign on / to sign

  • Hebrew uses the preposition על with this verb:
  • לחתום על המסמך = to sign the document

• למלא takes a direct object:

  • למלא את הטופס = to fill out the form

That is why:

• על המסמך has the preposition על
• את הטופס has the direct object marker את

A native English speaker often wants to say לחתום את המסמך, but standard Hebrew normally says לחתום על המסמך.

Yes. You could also say:

תמלאי את הטופס לאט לפני שאת חותמת על המסמך.

That is also grammatical.

Starting with לפני שאת חותמת על המסמך puts the time condition first, just like English Before you sign.... It sounds very natural.

The comma is also natural when that introductory clause comes first.

A simple transliteration is:

lifnei she'at chotemet al ha-mismakh, temal'i et ha-tofes le'at

A few pronunciation notes:

• שאת = she'at
• the pronoun את = at
• the object marker את = et
• לאט = le'at, not like English late

So the two written את are not only different in function, but also pronounced differently.