Para el desayuno, me gusta poner avena en el yogur con plátano.

Because gustar works differently from to like in English.

Spanish structures it more like something is pleasing to me:

• me = to me
• gusta = is pleasing

So me gusta poner avena... literally works like putting oats... is pleasing to me.

Yo gusto usually means I am pleasing / attractive, which is a different idea and not what you want here.

Because the thing being liked is the whole action poner avena en el yogur con plátano.

An infinitive phrase like poner avena... counts as singular, so Spanish uses gusta.

Compare:

• Me gusta poner avena en el yogur. = I like putting oats in yogurt.
• Me gustan los plátanos. = I like bananas.

So here, the subject is not avena or plátano by themselves, but the action poner.

Yes, literally poner means to put, but in food contexts it often means to add or to put into/on.

So poner avena en el yogur is a very natural way to say add oats to yogurt.

In Spain, you might also hear:

• echar avena en/al yogur
• añadir avena al yogur

But poner is perfectly normal here.

Para el desayuno means for breakfast or as a breakfast choice.

It focuses on the meal as the intended occasion.

A few useful comparisons:

• Para el desayuno... = for breakfast
• En el desayuno... = at breakfast / during breakfast
• Para desayunar... = for breakfast / for eating at breakfast

So in this sentence, para el desayuno is a natural way to introduce what the speaker likes to eat then.

Yes. Para desayunar is very natural and common.

Compare:

• Para el desayuno, me gusta poner avena en el yogur con plátano.
• Para desayunar, me gusta poner avena en el yogur con plátano.

Both are correct.
The version with para desayunar sounds a little more verbal and direct, while para el desayuno names the meal itself.

Because they are functioning differently.

el desayuno is a specific meal name in this structure, so the article is natural:

• para el desayuno
• durante la cena
• después del almuerzo

But avena is being used as a general ingredient or substance, so Spanish often leaves out the article:

• poner avena
• comprar pan
• beber agua

If you said la avena, it would usually sound more specific, like a particular oatmeal/oats already known in the conversation.

Yes. In standard Spanish, especially in Spain, yogur is masculine:

• el yogur
• un yogur

So en el yogur is exactly what you would expect.

You may occasionally hear other forms in some places, but for Spain, el yogur is the standard choice.

As written, con plátano most naturally goes with el yogur:

• el yogur con plátano = the yogurt with banana

So the sentence most directly suggests yogurt with banana, and the speaker likes to put oats into that.

If you want to make it clearer that both oats and banana are being added to the yogurt, Spanish could say:

• Me gusta poner avena y plátano en el yogur.

If you mean banana yogurt as a flavour, yogur de plátano might be clearer.

Yes. In Spain, plátano is the usual everyday word for banana.

In other Spanish-speaking countries, you may also hear:

• banana
• banano

But for Spanish from Spain, plátano sounds very natural.

Because the stress falls on the first syllable: PLÁ-ta-no.

Without the accent mark, normal Spanish stress rules would make it sound like pla-TA-no, which is wrong.

So the accent shows the correct pronunciation.

With poner, en el yogur is the most natural choice, because you are putting something in the yogurt.

• poner avena en el yogur

If you use verbs like añadir or echar, then al yogur becomes very natural:

• añadir avena al yogur
• echar avena al yogur

So both patterns exist, but they match different verbs.

Not strictly necessary, but it is natural.

The phrase Para el desayuno has been moved to the front as a topic-setting phrase, and the comma marks a small pause:

• Para el desayuno, me gusta...

You could also write:

• Para el desayuno me gusta...

Both are acceptable. The comma just makes the opening pause a bit clearer.