Assim que o afinares, testa a transmissão.

Assim que means as soon as.

It introduces an action that should happen before the action in the main clause. So the structure is basically:

As soon as X happens, do Y.

In Portuguese, assim que is very common in instructions and everyday speech.

Because after assim que, when you are talking about a future action, Portuguese normally uses the future subjunctive.

So:

• afinas = present indicative, you tune / you are tuning
• afinar = infinitive, to tune
• afinares = future subjunctive for tu, used here after assim que

That is why Assim que o afinares is the correct structure for As soon as you tune it.

It is the 2nd person singular form of the verb afinar in the future subjunctive.

The person is tu:

• quando eu afinar
• quando tu afinares
• quando ele/ela afinar

A useful thing to know: for many verbs, the future subjunctive and the personal infinitive can look the same. Here, even though afinares has the same shape, its function is future subjunctive, because it comes after assim que and refers to a future event.

This is about clitic pronoun placement, which is especially important in European Portuguese.

In European Portuguese, subordinate clauses introduced by conjunctions like assim que normally trigger proclisis, meaning the object pronoun goes before the verb.

So you get:

• Assim que o afinares...

not:

• Assim que afinares-o...

In a simple affirmative main clause, European Portuguese often prefers the pronoun after the verb:

• Afina-o. = Tune it.

But here, because the verb is inside a subordinate clause introduced by assim que, the pronoun moves before the verb.

O is a direct object pronoun meaning him or it.

In this sentence, it most likely means it and refers to some masculine singular noun mentioned earlier.

For example:

• o = masculine singular
• a = feminine singular
• os = masculine plural
• as = feminine plural

So if the thing being adjusted were feminine, the sentence would use a instead.

Yes. Testa here is the affirmative imperative for tu.

With most verbs, the affirmative imperative for tu is formed from the present indicative by dropping the final -s:

• tu testas → testa
• tu afinas → afina

So testa a transmissão means test the transmission when speaking informally to one person.

Because testa is the command form for tu, while teste is used for você or other more formal/polite forms.

Compare:

• tu → testa
• você / o senhor / a senhora → teste

So this sentence is clearly addressing someone with tu, not você.

Because Portuguese often leaves subject pronouns out when the verb form already makes the meaning clear.

Here, afinares clearly points to tu, and testa also fits the tu command in this context.

So Portuguese prefers:

• Assim que o afinares, testa a transmissão.

rather than:

• Assim que tu o afinares, tu testa a transmissão.

You can add tu for emphasis, but normally it is omitted.

A more formal version would be:

Assim que o afinar, teste a transmissão.

What changed:

• afinares → afinar because this is the 3rd person singular future subjunctive, used with você
• testa → teste because that is the command form for você

So:

• informal tu: Assim que o afinares, testa a transmissão.
• formal você: Assim que o afinar, teste a transmissão.

Yes, it sounds very natural in European Portuguese.

Two things especially stand out:

• the use of tu
• the pronoun placement in o afinares

In Brazilian Portuguese, you might more often hear something like:

• Assim que você o afinar, teste a transmissão.

or other small variations depending on formality and region.

So the original sentence is a good example of standard European Portuguese grammar and style.