Добавь немного перца в суп, но не добавляй много соли.

They are imperatives of two different aspects of the verb добавлять/добавить:

• Добавь = perfective imperative (from добавить): tells you to do a single, completed action (add it once, get it done).
• Не добавляй = imperfective imperative (from добавлять): tells you not to do an action in general / not to engage in it (don’t add / don’t go adding).

With prohibitions, Russian most often uses the imperfective: Не делай X = don’t do X / don’t start doing X / don’t be doing X.
Не добавь is possible but much less common and usually sounds like “don’t accidentally add (it)” or “make sure you don’t add (it)”—more like a warning about a single mishap. In a cooking instruction, the natural neutral prohibition is не добавляй.

Words meaning an indefinite quantity—немного, много, мало, столько, etc.—normally require the genitive:

• немного + Gen → немного перца
• много + Gen → много соли

This is especially typical with substances/mass nouns like pepper and salt.

В суп uses в + accusative because it expresses movement/adding something into the soup (direction/result):

• в + Acc = into (motion/direction) → в суп
• в + Prep = in/inside (location) → в супе (meaning “in the soup,” location)

Here the action is putting pepper into the soup, so в суп is used.

Yes, effectively. With ingredients/substances, the genitive after quantity words often has a partitive feel: “a bit of pepper,” “a lot of salt.” It’s not necessarily “part of a particular pepper,” just the normal way Russian expresses amounts of substances.

Yes, in this sentence it’s standard. Но connects two independent imperative clauses:

• Добавь немного перца в суп,
• но не добавляй много соли.

Russian normally puts a comma before но when it joins two clauses like this.

Yes, that’s very common and still natural:

• Добавь немного перца в суп (focus slightly more on “a bit of pepper”)
• Добавь в суп немного перца (focus slightly more on “into the soup”)

Russian word order is flexible; changes usually shift emphasis, not basic meaning.

They come from the aspect pair:

• добавить (perfective) → imperative добавь
• добавлять (imperfective) → imperative добавляй

In dictionaries you’ll often see them as a pair: добавлять/добавить = “to add.”

Use the -те ending:

• Добавьте немного перца в суп, но не добавляйте много соли.

That’s the polite/plural version addressed to вы.

Соль is usually treated as a mass noun in Russian, so quantities are expressed with genitive singular:

• много соли = a lot of salt

Plural (соли) exists in other meanings (e.g., “salts” in chemistry), but for table salt as an ingredient the mass-noun pattern is normal.

Yes, very naturally:

• немного → чуть-чуть, немножко (a little bit)
• много → слишком много (too much), многовато (a bit too much)

Example:

• Добавь чуть-чуть перца в суп, но не добавляй слишком много соли.