Impermeabilizziamo la parete affinché l'intonaco non cada.

Impermeabilizziamo is the 1st person plural form of impermeabilizzare, so it means we waterproof.

In a sentence like this, it is most naturally understood as the present indicative: We waterproof the wall so that the plaster does not fall.

Italian forms like impermeabilizziamo can sometimes also mean let's waterproof, because the 1st person plural is also used for the imperative. Usually, context tells you which meaning is intended.

Italian often does not state the subject pronoun when the verb ending already makes it clear.

Here, -iamo shows that the subject is we, so noi is unnecessary. You could say Noi impermeabilizziamo la parete..., but that would usually add emphasis, such as we (and not someone else).

La parete usually means the wall in the sense of a surface/partition of a room or building.

Il muro often suggests a more solid wall structure, such as an exterior wall, boundary wall, or masonry wall.

In many contexts they can both be translated as wall, but parete is often the more natural choice when talking about the wall as a surface that is treated, painted, plastered, or waterproofed.

Affinché means so that, in order that, or so as to ensure that.

It introduces a clause of purpose:

• Impermeabilizziamo la parete affinché l'intonaco non cada.
• We waterproof the wall so that the plaster does not fall.

It is a bit more formal than some other ways of expressing purpose, and it normally requires the subjunctive in the following clause.

Because affinché is followed by the subjunctive, not the indicative.

The verb cadere has:

• cade = present indicative, he/she/it falls
• cada = present subjunctive, that it fall

So:

• affinché l'intonaco non cada = so that the plaster does not fall

This is one of the most important grammar points in the sentence: purpose expressions like affinché trigger the subjunctive.

In Italian, non goes directly before the verb to make it negative.

So:

• cade = it falls
• non cade = it does not fall
• cada = that it fall
• non cada = that it not fall / that it does not fall

In this sentence, non negates the verb in the subordinate clause:

• affinché l'intonaco non cada = so that the plaster does not fall

Intonaco is a masculine singular noun meaning plaster or render, depending on context.

It appears here as l'intonaco because il becomes l' before a vowel:

• il muro
• l'intonaco

So l'intonaco still means the plaster; the apostrophe is just the normal elision before a vowel.

Sometimes yes, but there is an important nuance.

Perché can also mean so that / in order that, and when it has that meaning, it also takes the subjunctive:

• Impermeabilizziamo la parete perché l'intonaco non cada.

That is grammatical.

However:

• affinché is more explicitly and clearly a purpose conjunction
• perché can also mean because, so it can sometimes be more ambiguous
• affinché sounds more formal or careful in written Italian

So in this sentence, affinché is a very good choice.

It is used transitively, so it takes a direct object with no preposition:

• impermeabilizzare la parete = to waterproof the wall

That is why the sentence says:

• Impermeabilizziamo la parete...

and not something like alla parete.

The accent mark shows that the stress falls on the final syllable: affin-ché.

This written accent is normal in Italian for certain words stressed on the last syllable. It also helps distinguish the correct spelling.

So the word is written:

• affinché

not affinche.

Yes, it can, depending on context.

Because impermeabilizziamo can be either:

• we waterproof (present indicative)
• let's waterproof (1st person plural imperative)

Without more context, both are possible. In isolation, many learners first read it as we waterproof, but in a practical or instructional context it could easily mean let's waterproof the wall so that the plaster doesn't fall.