Se l’impasto è troppo duro, aggiungo un po’ di acqua.

Yes. Se is the normal word for if in conditional sentences.

In Se l’impasto è troppo duro, aggiungo un po’ di acqua, the pattern is:

• Se
  • present tense
• present tense in the main clause

This is a very common way to talk about a real, possible situation:

• Se piove, resto a casa.
• Se ho tempo, ti chiamo.

So here it means something like: in that situation, this is what I do.

Because il becomes l’ before a vowel.

• il pane
• il forno
• l’impasto
• l’amico

So l’impasto is just il impasto contracted for easier pronunciation.

Also, impasto is masculine singular, so its full article is il, not la.

Because è is the verb essere in the he/she/it is form.

The accent is important because it distinguishes è from e:

• è = is
• e = and

So:

• L’impasto è duro = The dough is hard
• farina e acqua = flour and water

Because duro agrees with l’impasto, which is masculine singular.

Italian adjectives usually agree in gender and number with the noun they describe:

• masculine singular: duro
• feminine singular: dura
• masculine plural: duri
• feminine plural: dure

Since impasto is masculine singular, the adjective must also be masculine singular: duro.

Here troppo means too and modifies the adjective duro.

So:

• duro = hard
• troppo duro = too hard

This is a very common pattern:

• troppo caldo = too hot
• troppo lento = too slow
• troppo difficile = too difficult

Because Italian often drops subject pronouns when they are not needed.

The ending -o in aggiungo already tells you the subject is I:

• aggiungo = I add
• aggiungi = you add
• aggiunge = he/she adds

So io is usually omitted unless you want emphasis or contrast:

• Io aggiungo acqua, tu invece no.

Without emphasis, aggiungo by itself is completely normal.

Italian often uses the present tense where English may also use the present tense or sometimes a future-like idea depending on context.

In sentences with se, Italian very often uses the present for real conditions:

• Se è pronto, lo mangio.
• Se ho fame, cucino.

So Se l’impasto è troppo duro, aggiungo... means this is what I do whenever that condition happens.

It can sound like:

• a general habit
• a cooking instruction
• a step in a recipe explained personally

Italian does not normally use a future after se in this kind of sentence.

Un po’ means a little, a bit.

It comes from poco, but in this expression the word is shortened:

• poco → po’

That is why you write po’ with an apostrophe.

Examples:

• un po’ di zucchero
• un po’ stanco
• aspetta un po’

So here un po’ di acqua means a little water.

Yes, both are possible.

You will very often hear and see:

• un po’ d’acqua

because di is commonly shortened to d’ before a vowel.

But un po’ di acqua is also understandable and acceptable.

So:

• un po’ di acqua = fine
• un po’ d’acqua = very common, often more natural-sounding

After un po’ di, the noun usually appears without an article:

• un po’ di farina
• un po’ di sale
• un po’ d’acqua

The comma separates the if-clause from the main clause.

• Se l’impasto è troppo duro, aggiungo un po’ di acqua.

This is very common when the se clause comes first.

If you reverse the order, the comma is often not needed:

• Aggiungo un po’ di acqua se l’impasto è troppo duro.

So the comma here is natural and helps readability.

Yes.

You can say:

• Se l’impasto è troppo duro, aggiungo un po’ di acqua.
• Aggiungo un po’ di acqua se l’impasto è troppo duro.

Both are correct.

Putting the se clause first slightly highlights the condition. Putting it second starts with the action.

Yes. After expressions of quantity like un po’ di, Italian normally uses di + noun without an article.

So you say:

• un po’ di acqua
• un litro di latte
• molta farina
• poco sale

You would not normally say un po’ dell’acqua unless you mean some of the water, referring to specific water already mentioned.

Yes, it can be.

This sentence sounds natural if someone is describing what they do while cooking or giving informal recipe guidance from a personal point of view.

Italian recipes can also use other styles, for example:

• aggiungere un po’ d’acqua — infinitive style
• aggiungete un po’ d’acqua — plural imperative
• aggiungi un po’ d’acqua — singular imperative

So aggiungo is not the only possible form, but it is perfectly natural if the speaker is explaining their own method.