Se houver outra mancha na camisa, eu vou deixá-la de molho antes de colocar na máquina.

Because se often triggers the future subjunctive when you are talking about a possible future condition.

So:

• Se houver outra mancha... = If there is / if there happens to be another stain...

Here, houver is the future subjunctive form of haver.

A quick comparison:

• Há uma mancha na camisa. = There is a stain on the shirt.
• Se houver uma mancha na camisa... = If there is a stain on the shirt...

So há states a fact now, while houver is used in a possible future situation after se.

Yes, in Brazilian Portuguese, that is very common in everyday speech.

• Se houver outra mancha na camisa... sounds more formal or more written.
• Se tiver outra mancha na camisa... sounds more conversational and natural in many spoken contexts.

Both can mean essentially the same thing here.

A learner should recognize both, but houver is useful because it appears often in writing and more standard phrasing.

Outra means another or other.

So outra mancha means:

• another stain
• one more stain

It suggests there may already be one stain known or expected, and the speaker is talking about the possibility of an additional one.

Because na is a contraction of em + a.

So:

• em a camisa becomes na camisa
• em a máquina becomes na máquina

This is extremely common in Portuguese.

Other examples:

• no = em + o
• nas = em + as
• nos = em + os

So:

• na camisa = on/in the shirt
• na máquina = in the machine

Because -la is an attached object pronoun.

Here:

• deixar = to leave
• a = her/it
• deixar + a becomes deixá-la

This spelling change happens because when -la is attached to an infinitive ending in -r, the -r drops, a hyphen is added, and the vowel often gets an accent:

• deixar + a → deixá-la
• colocar + a → colocá-la
• lavar + o → lavá-lo

This is standard written Portuguese.

It refers to camisa.

Even though both mancha and camisa are feminine singular, the meaning makes it clear:

• You do not normally soak the stain
• You soak the shirt

So:

• deixá-la de molho = leave the shirt soaking

This is a good example of Portuguese using a pronoun whose referent has to be understood from context.

De molho means soaking or left to soak.

So:

• deixar algo de molho = to leave something soaking
• a camisa está de molho = the shirt is soaking / is being soaked

It is a very common expression in everyday Portuguese, especially for clothes, beans, and dishes.

Examples:

• Vou deixar o feijão de molho. = I’m going to soak the beans.
• Deixei a roupa de molho. = I left the clothes soaking.

Because ir + infinitive is extremely common in Brazilian Portuguese, especially in speech.

So:

• eu vou deixar = I’m going to leave / I will leave
• deixarei = I will leave

Both are correct, but vou deixar sounds more natural in everyday Brazilian Portuguese.

In general:

• vou fazer
• vou lavar
• vou colocar

are usually more common in conversation than:

• farei
• lavarei
• colocarei

Because Portuguese often omits the object when it is already obvious from context.

Here, the listener already knows the object is the shirt, so colocar na máquina naturally means:

• put it in the machine

A fuller version would be:

• antes de colocá-la na máquina

That is also correct.

So the sentence could be either:

• ...antes de colocar na máquina
• ...antes de colocá-la na máquina

The shorter version sounds very natural because the object is understood.

Yes, literally it means in the machine, but in this context it clearly means in the washing machine.

In Portuguese, especially in everyday speech, people often shorten:

• máquina de lavar
• máquina de lavar roupa

to simply:

• máquina

So in a laundry context, colocar na máquina means put it in the washing machine.

In everyday Brazilian speech, many people would naturally say deixar ela.

So you may hear:

• vou deixar ela de molho

This is very common in spoken Brazilian Portuguese.

However:

• vou deixá-la de molho

is the more standard written form and is often taught to learners.

So the difference is roughly:

• deixá-la = more standard, more formal, common in writing
• deixar ela = very common in speech, more colloquial

Both are useful to know.

No, it is not necessary.

Portuguese often drops subject pronouns when the verb already makes the subject clear. So both are correct:

• eu vou deixá-la de molho
• vou deixá-la de molho

The speaker may include eu for emphasis, clarity, or just personal speaking style.

So here, eu is optional rather than required.