אחרי המקלחת אני שמה קרם, ואחר כך משתמשת בשמפו שהאחות שלי המליצה עליו.

Because the speaker is female.

In Hebrew, many verbs in the present tense agree with the gender of the subject:

• masculine singular: שם
• feminine singular: שמה

So:

• אני שם קרם = I put/apply cream (said by a male speaker)
• אני שמה קרם = I put/apply cream (said by a female speaker)

The same thing happens later in the sentence with משתמשת, which is also feminine singular.

Literally, שמה comes from the verb לשים = to put.

But in everyday Hebrew, לשים קרם is a very natural way to say:

• to put on cream
• to apply cream

So אני שמה קרם means I apply cream or I put on cream, not just I place cream somewhere.

Because Hebrew often uses the definite article ה־ where English might not.

• מקלחת = a shower
• המקלחת = the shower

So אחרי המקלחת literally means after the shower.

In context, this often works like English after showering or after my shower. Hebrew commonly says אחרי המקלחת even when English would be less definite.

Both relate to time, but they do different jobs.

• אחרי = after and is usually followed by a noun or phrase

  • אחרי המקלחת = after the shower

• אחר כך = afterward / then / after that

  • ואחר כך משתמשת... = and afterward / and then I use...

So in this sentence:

• אחרי המקלחת = after the shower
• ואחר כך = and then / afterward

Because the verb להשתמש requires the preposition ב־ = in / with / by, but in English we usually just say use something directly.

So in Hebrew:

• להשתמש ב... = to use ...

Examples:

• אני משתמשת בשמפו = I use shampoo
• הוא משתמש בטלפון = He uses the phone
• אנחנו משתמשים במחשב = We use a computer / the computer

Here ב + שמפו becomes בשמפו.

Because Hebrew often attaches prepositions directly to the following word.

• ב־ = in / with / by
• שמפו = shampoo

So:

• ב + שמפו = בשמפו

This is very common in Hebrew. For example:

• בבית = in the house
• בספר = in the book
• בקרם = with cream / in the cream depending on context

In this sentence, בשמפו simply means shampoo as the object of use, because להשתמש needs ב־.

It is a relative clause describing השמפו.

The structure is:

• השמפו = the shampoo
• שהאחות שלי המליצה עליו = that my sister recommended

Together:

• השמפו שהאחות שלי המליצה עליו = the shampoo that my sister recommended

The word ש־ here means that / which and introduces the clause.

Because the verb להמליץ usually works with the preposition על when you recommend something.

So Hebrew says:

• להמליץ על משהו = to recommend something
  more literally, to recommend about/on something

Examples:

• היא המליצה על ספר = She recommended a book
• הוא המליץ על מסעדה = He recommended a restaurant

In your sentence, the object being recommended is השמפו, and in the relative clause Hebrew keeps the preposition:

• השמפו ... המליצה עליו
  literally: the shampoo ... she recommended on it

This sounds odd in English, but it is normal in Hebrew.

Because the verb להמליץ does not normally take a direct object. It takes על.

Compare:

• אני רואה אותו = I see him/it
  Here לראות takes a direct object, so אותו works.

But:

• אני ממליצה עליו = I recommend it / him
  because להמליץ needs על

So in the relative clause, Hebrew must say:

• שהאחות שלי המליצה עליו

not:

• שהאחות שלי המליצה אותו

עליו means on him / on it.

It is made of:

• על = on / about
• ־יו = him / it masculine singular

It is masculine here because שמפו is grammatically masculine.

So:

• שמפו → עליו
• if the noun were feminine, you would use עליה

For example:

• המסכה שהרופאה המליצה עליה = the mask that the doctor recommended

That is just how possession often works in Hebrew.

• האחות = the sister
• שלי = my

So האחות שלי = my sister

Hebrew commonly expresses possession with של:

• הספר שלי = my book
• החבר שלה = her friend
• הבית שלנו = our house

Even though it looks a bit different from English, it is one of the most basic and common possession patterns in Hebrew.

Yes, mostly.

The main actions are in the present tense:

• אני שמה = I put / I apply
• משתמשת = I use / am using depending on context

But the relative clause contains a past-tense verb:

• המליצה = recommended

So the sentence mixes tenses naturally:

• present: what the speaker usually does now
• past: what the sister did earlier

This is completely normal: After the shower I put on cream, and then I use the shampoo that my sister recommended.

Not exactly. A male speaker would usually change the feminine present-tense forms to masculine:

• אחרי המקלחת אני שם קרם, ואחר כך משתמש בשמפו שהאחות שלי המליצה עליו.

Changes:

• שמה → שם
• משתמשת → משתמש

Everything else can stay the same.

In standard usage, שמפו is usually treated as masculine.

That matters because pronouns and some related words may agree with it. In this sentence, that is why we get:

• עליו = on it masculine

If the noun were feminine, the pronoun would change accordingly.

Also, notice that שמפו itself does not visibly change here, but the pronoun referring back to it does.