חשוב שתסחטי לימון טרי על הסלט לפני שאת שמה בו חסה ופטריות.

The ש־ is a conjunction meaning something like that. After חשוב, Hebrew commonly uses ש־ plus a verb:

חשוב שתסחטי = it’s important that you squeeze

So שתסחטי is literally that you will squeeze / that you squeeze, depending on how natural English renders it.

In Hebrew, the future form is often used after words like חשוב, כדאי, רצוי, and similar expressions to give advice, instructions, or recommendations.

So חשוב שתסחטי does not have to mean strict future time. It means something more like:

• it’s important that you squeeze
• you should squeeze

This is a very normal Hebrew pattern.

Because the sentence is addressing one female person.

You can see that in:

• תסחטי = you (feminine singular) will squeeze / should squeeze
• את = you (feminine singular)
• שמה = putting / puts in feminine singular

If you were speaking to a man, you would change the forms, for example:

• חשוב שתסחט...
• לפני שאתה שם...

שאת is a combination of ש־ + את.

Here:

• ש־ introduces the clause
• את means you (feminine singular)

So לפני שאת שמה literally looks like before that you put, but in natural English it simply means before you put.

Also, שמה is the present feminine singular form of לשים (to put). So:

• את שמה = you put / you are putting

In this sentence, that whole part means before you put.

Yes. Both are possible.

• לפני שאת שמה בו חסה ופטריות
• לפני שתשימי בו חסה ופטריות

Both mean before you put lettuce and mushrooms in it.

The version with לפני שאת שמה is very common in everyday spoken Hebrew. The version with לפני שתשימי is also perfectly good and may sound a bit more compact.

בו means in it.

It is made from:

• ב־ = in
• הוא = he/it for a masculine noun

Here it refers back to הסלט (the salad), which is a masculine singular noun in Hebrew.

So:

• שמה בו חסה ופטריות = puts lettuce and mushrooms in it

Because the relationships are different:

• לסחוט לימון על הסלט = to squeeze lemon onto / over the salad
• לשים חסה ופטריות בו = to put lettuce and mushrooms in it

So Hebrew is making the same distinction English usually makes:

• dressing, juice, sauce, etc. go on / over a salad
• ingredients go in a salad

Because Hebrew uses the direct object marker את only with definite direct objects.

Here:

• לימון טרי = a fresh lemon / fresh lemon → indefinite
• חסה ופטריות = lettuce and mushrooms → indefinite

So there is no את before them.

Compare:

• סחטי לימון = squeeze a lemon
• סחטי את הלימון = squeeze the lemon

Because חסה often works like a mass noun, similar to English lettuce. English also usually says add lettuce, not add lettuces.

But פטריות is plural because mushrooms are usually counted as separate pieces:

• חסה = lettuce
• פטריות = mushrooms

So the mix of singular and plural is completely natural.

Literally, לימון טרי is fresh lemon. But with the verb לסחוט (to squeeze), the idea is naturally squeeze fresh lemon juice onto the salad.

Hebrew often mentions the fruit itself rather than explicitly saying מיץ לימון (lemon juice) in this kind of sentence.