Learn Hardcore Portuguese (Portugal): Quando o pólen entra em casa, eu começo a espirrar sem parar. - When pollen comes into the house, I start sneezing non-stop.

Quando o pólen entra em casa, → subordinate time clause (“When pollen comes into the house”)
eu começo a espirrar sem parar. → main clause (“I start sneezing non-stop.”)

If you reversed the order, you would usually drop the comma:

Eu começo a espirrar sem parar quando o pólen entra em casa.
(No comma in the middle.)

Is the subject pronoun eu necessary here?

No, it’s not grammatically necessary. Portuguese is a null-subject language, so you can usually omit the subject pronoun when the verb ending is clear:

(Eu) começo a espirrar sem parar.

Both are correct. In European Portuguese:

Omitting eu is very common in neutral speech: Quando o pólen entra em casa, começo a espirrar sem parar.
Including eu can add a bit of emphasis or clarity, or just be the speaker’s style: … eu começo a espirrar …

Why is it em casa and not na casa?

Em casa is a set expression meaning “at home/into the house/inside the house,” without any article:

em casa = at home, in the house (in general)
na casa = in the house (a specific house that has already been identified)

The sentence talks about pollen entering the home in a general sense, not a particular house that’s been specified in the conversation, so em casa is the natural choice.

Compare:

Quando o pólen entra em casa, eu começo a espirrar.
= When pollen gets into my home, I start sneezing.
Quando o pólen entra na casa da Ana, o cão dela também espirra.
= When pollen gets into Ana’s house, her dog also sneezes.
(Here na casa refers to a particular house, Ana’s.)

Why is it entra em casa and not something like vai para casa?

Entrar em focuses on entering / coming inside a place:

entrar em casa = come into the house / enter the house

Ir para casa is “go home,” with emphasis on going towards a place, not necessarily crossing the door/threshold at that moment.

Since pollen is coming inside the house, entrar em casa is the natural verb + preposition combination here.

Why do we say começo a espirrar and not just começo espirrar?

In Portuguese, when começar is followed by a verb in the infinitive, you almost always need the preposition a:

começar a + infinitive = to start doing something

So:

começo a espirrar = I start sneezing
começamos a trabalhar = we start working
eles começaram a rir = they started laughing

Without a, it sounds ungrammatical:
✗ começo espirrar is wrong.

What does sem parar mean exactly?

Literally, sem parar means “without stopping.”
Idiomatic meaning: non-stop / continuously / over and over.

So eu começo a espirrar sem parar is:

literally: “I start to sneeze without stopping.”
natural English: “I start sneezing non-stop” or “I can’t stop sneezing.”

You can use sem parar after many verbs:

falar sem parar – talk non-stop
chover sem parar – rain continuously
correr sem parar – run without stopping

Why is the present tense (entra, começo) used, if this is a repeated or habitual action?

In Portuguese, the simple present is used for:

current actions,
general truths,
habits and repeated actions.

So even if this is a habitual reaction (“Whenever pollen comes in, I start sneezing”), you still use the present:

Quando o pólen entra em casa, eu começo a espirrar.

You don’t need any extra word like “usually” or “whenever”; the structure with quando + present already gives a habitual meaning.

Why do we say o pólen with the article o?

Portuguese normally uses the definite article with general or abstract nouns more than English does.

o pólen literally: “the pollen”
but it often corresponds to English “pollen” in a general sense.

So:

Quando o pólen entra em casa, eu começo a espirrar.
= “When pollen gets in the house, I start sneezing.”

Leaving out the article:

✗ Quando pólen entra em casa… is not natural Portuguese. You need o pólen here.

Is pólen masculine or feminine? And is it countable?

pólen is masculine: o pólen.
It is usually treated as a mass noun, like “sand” or “water” in English.

You would normally say:

Há muito pólen no ar. – There is a lot of pollen in the air.

Using a plural os pólens is very rare and generally unnecessary outside scientific/technical contexts talking about types of pollen.

Could we say eu não paro de espirrar instead of eu começo a espirrar sem parar?

You could, but the meaning changes slightly:

eu começo a espirrar sem parar
– focus on the moment it starts, and then it keeps going.
eu não paro de espirrar
– focus on the fact that you cannot stop, implying it has already been going on for a while.

Both are natural and could be used in similar contexts, but they are not identical in nuance.

Is there another way to say espirrar in Portuguese, or another common expression for sneezing?

The basic verb is espirrar – “to sneeze.”

A very common alternative expression is:

dar espirros – literally “to give sneezes.”

Examples:

Eu começo a espirrar sem parar.
Eu começo a dar espirros sem parar.

Both are natural. Espirrar is shorter and more direct; dar espirros feels slightly more descriptive.

Is the word order eu começo a espirrar fixed, or can it be changed?

The normal order is:

subject + verb + rest of the sentence
→ eu começo a espirrar sem parar.

You will see other orders, but they are less neutral:

Começo eu a espirrar sem parar. – possible in written or emphatic speech, but sounds marked / poetic / stylistic.
Eu sem parar começo a espirrar. – sounds strange and unnatural.

For everyday European Portuguese, eu começo a espirrar sem parar (or dropping eu) is the standard word order.