Quan té al·lèrgia, comença a esternudar i busca aigua.

Catalan often omits subject pronouns because the verb ending usually already tells you the person.

• té = he/she/it has
• comença = he/she/it begins
• busca = he/she/it looks for

So the sentence naturally leaves out ell/ella. This is very common in Catalan, just like in Spanish or Italian.

It can often feel like either, depending on context.

In this sentence, quan introduces a time clause:

• Quan té al·lèrgia = When / Whenever he or she has an allergy

If the sentence is describing a usual reaction, English might prefer whenever:

• Whenever he has allergies, he starts sneezing and looks for water.

If it refers to a specific occasion, when also works.

Catalan often leaves out the article with some conditions or states, especially when speaking in a general way.

So:

• tenir al·lèrgia = to have an allergy / to be allergic
• tenir una al·lèrgia would sound more like to have an allergy of a particular kind or a specific allergy

In everyday language, té al·lèrgia sounds natural if the idea is general.

Té is the 3rd person singular present form of tenir (to have).

Conjugation of tenir in the present:

• jo tinc = I have
• tu tens = you have
• ell/ella té = he/she has
• nosaltres tenim = we have
• vosaltres teniu = you all have
• ells/elles tenen = they have

So té means has.

Because in Catalan, començar is usually followed by a + infinitive when it means to begin/start doing something.

So:

• comença a esternudar = starts to sneeze
• començo a llegir = I start reading
• van començar a parlar = they started talking

This is a very common pattern:

• començar a + infinitive

Yes. Esternudar means to sneeze.

Examples:

• Esternuda molt = He/She sneezes a lot
• Ha començat a esternudar = He/She has started sneezing

In your sentence:

• comença a esternudar = starts sneezing / starts to sneeze

In Catalan, buscar is normally a direct transitive verb, so it takes its object directly, with no preposition.

• busca aigua = looks for water
• not busca per aigua

Also, aigua appears without an article here because it means water in general, not a specific glass or bottle already known to the listener.

Compare:

• busca aigua = looking for some water
• busca l’aigua = looking for the water, a specific water already identified in context

Here aigua is singular and uncountable, just like water in English.

• aigua = water
• aigües can exist, but it usually means waters in special contexts, such as types of water, mineral waters, or geographical/literary uses

In normal everyday speech, if someone wants something to drink, aigua is the natural form.

All the verbs are in the present indicative:

• té
• comença
• busca

The present tense in Catalan can describe:

1. a current event
2. a habitual action
3. a general truth or usual reaction

Here it most naturally sounds like a habitual or typical reaction:

• When/Whenever he has an allergy, he starts sneezing and looks for water.

It breaks down like this:

• Quan té al·lèrgia = time clause
• comença a esternudar = first main action
• i busca aigua = second main action

So the pattern is:

When + situation, action 1 and action 2.

That makes the sentence easy to expand:

• Quan està nerviós, parla molt i camina pel passadís.
• Quan té fred, es posa l’abric i tanca la finestra.

The accent mark shows stress and sometimes helps distinguish words.

• té has an accent to show the stressed vowel and to distinguish it from te (tea)
• al·lèrgia has the stress on the syllable with lèr

So:

• té ≠ te
• al·lèrgia is stressed roughly like a-LLÈR-gi-a

Accents are important in Catalan and should be learned as part of the word.

That dot is called the punt volat. It appears in l·l.

Its job is to show that these are two separate l sounds, not the digraph ll.

So:

• ll is a special Catalan sound
• l·l means the two l letters are pronounced separately

Examples:

• al·lèrgia
• col·legi
• il·legal

This is a spelling feature you will see in some Catalan words of learned or Latin origin.

Yes, depending on the meaning.

• Quan té al·lèrgia... = when he/she has an allergy / is having an allergic reaction
• Quan té al·lèrgies... = when he/she has allergies

The plural often sounds more natural if you mean a general recurring condition:

• Quan té al·lèrgies, comença a esternudar i busca aigua.

The singular is still understandable, but the plural may fit better in some contexts.

Yes, it is understandable and basically natural. A native speaker might slightly adjust it depending on the exact meaning.

Possible alternatives:

• Quan té al·lèrgia, comença a esternudar i busca aigua.
• Quan té al·lèrgies, comença a esternudar i busca aigua.
• Quan té una reacció al·lèrgica, comença a esternudar i busca aigua.

The best version depends on whether you mean:

• a general allergy condition
• allergies in general
• a specific allergic reaction

But as a learning sentence, the original works well.