הוא הוסיף תירס לסלט, אבל אני ביקשתי שלא יוסיף פטריות.

Because after a verb like ביקשתי (I asked/requested), Hebrew usually uses the future form to express the action that someone was asked to do or not do.

So:

• ביקשתי שלא יוסיף פטריות = I asked that he not add mushrooms
• not ביקשתי שלא הוסיף, which would sound like you are talking about a past fact rather than a request

In this kind of sentence, the future form יוסיף works a bit like English should add / would add / add in a subordinate clause.

Here שלא means something like that not or so that not.

It is made from:

• ש־ = that
• לא = not

So:

• ביקשתי שלא יוסיף פטריות literally means I asked that he not add mushrooms

This is a very common Hebrew structure after verbs of asking, wanting, ordering, and so on.

Because לא only negates a verb, while שלא also connects the clause to what came before.

Compare:

• ביקשתי שלא יוסיף פטריות = I asked that he not add mushrooms
• לא יוסיף פטריות = he will not add mushrooms

So in your sentence, שלא is needed because the second clause depends on ביקשתי.

A related alternative is:

• ביקשתי לא להוסיף פטריות = I asked not to add mushrooms

But that version can sound less explicit about who is supposed to do the adding. With שלא יוסיף, it is clear that he is the one being asked not to add them.

Because Hebrew often leaves out subject pronouns when the verb already shows who the subject is.

• יוסיף already means he will add / he would add / he add in this kind of construction

So שלא יוסיף is enough to mean that he not add.

You could say שלא הוא יוסיף, but that would sound emphatic or unnatural in most normal contexts.

They are two forms of the same verb:

• הוסיף = past, he added
• יוסיף = future, he will add

This verb is from the root י-ס-ף, connected with adding/increasing.

In this sentence you get both forms:

• הוא הוסיף תירס לסלט = he added corn to the salad
• שלא יוסיף פטריות = that he not add mushrooms

So the sentence nicely shows both the past and future forms of the same verb.

Because Hebrew combines certain prepositions with the definite article ה־.

Here:

• ל־ = to
• הסלט = the salad

Together they become:

• לסלט = to the salad

This contraction is completely normal in Hebrew.

Similar examples:

• לבית = to the house
• למורה = to the teacher

So לסלט literally means to the salad.

Because את is usually used before a definite direct object.

Here, תירס and פטריות are not definite:

• תירס = corn
• פטריות = mushrooms

They do not have ה־ and they are not clearly marked as specific, so את is not used.

Compare:

• הוא הוסיף תירס = he added corn
• הוא הוסיף את התירס = he added the corn

and:

• שלא יוסיף פטריות = that he not add mushrooms
• שלא יוסיף את הפטריות = that he not add the mushrooms

The ending ־תי marks I in the past tense.

So:

• ביקשתי = I asked
• ביקשת = you asked (masculine singular)
• ביקשה = she asked
• ביקש = he asked

This is one of the most useful Hebrew past-tense endings to learn early, because it appears in many verbs.

פטריות is simply the plural noun mushrooms.

• singular: פטרייה = mushroom
• plural: פטריות = mushrooms

It does not affect the verb here, because the verb agrees with the subject, not with the object.

The subject of יוסיף is he, so the verb is masculine singular:

• שלא יוסיף פטריות = that he not add mushrooms

Even though פטריות is plural, the verb stays singular because he is doing the action.

Yes, absolutely. That is a very natural alternative.

It means roughly the same thing:

• ביקשתי שלא יוסיף פטריות
• ביקשתי ממנו לא להוסיף פטריות

The difference is mostly in structure:

1. ביקשתי שלא יוסיף פטריות

   • literally: I asked that he not add mushrooms
   • more clause-based

2. ביקשתי ממנו לא להוסיף פטריות

   • literally: I asked him not to add mushrooms
   • more explicit because of ממנו = from him / him

Both are good Hebrew.

Yes, slightly.

• אבל ביקשתי... = but I asked...
• אבל אני ביקשתי... = but I asked...

Adding אני gives extra emphasis or contrast. It can sound like:

• but I had asked...

In your sentence, אבל אני ביקשתי highlights the contrast between what he did and what I had asked for.

Yes, it is very normal.

The sentence is:

• הוא הוסיף תירס לסלט, אבל אני ביקשתי שלא יוסיף פטריות.

It has two main parts:

1. הוא הוסיף תירס לסלט
   = He added corn to the salad

2. אבל אני ביקשתי שלא יוסיף פטריות
   = But I asked that he not add mushrooms

This is a very standard Hebrew sentence pattern:

• subject + verb + object/complement
• then a second clause introduced by אבל

So a learner should think of it as natural, everyday Hebrew word order.