El maig passat no vam sentir cap tro, però a l'abril sí que hi va haver una tempesta.

In Catalan, months are often used with the definite article, especially when talking about a specific month in time.

So:

• el maig = May
• el maig passat = last May

This is very natural in Catalan. English usually says last May without an article, but Catalan commonly uses el.

Also, passat comes after the noun:

• el maig passat = last May
• literally: the May past

Vam sentir is the very common spoken and written past tense in Catalan, called the periphrastic past.

It is formed with:

• a form of anar (vaig, vas, va, vam, vau, van)
• plus the infinitive

So:

• vam sentir = we heard / we felt

This tense is extremely common in modern Catalan.
The simple past form sentírem also exists, but it is much less common in everyday language.

Yes, sentir can mean both to feel and to hear, depending on context.

In this sentence:

• vam sentir cap tro = we didn’t hear any thunder

Because tro means thunderclap / thunder, the meaning is clearly about hearing.

So sentir can cover:

• feel something physically or emotionally
• hear a sound

Cap is often used in negative sentences to mean any or no.

So:

• no vam sentir cap tro = we didn’t hear any thunder
• literally: we didn’t hear any thunderclap

This is a very common Catalan structure:

• No tinc cap problema = I don’t have any problem
• No ve ningú / No hi ha cap persona = No one is coming / There isn’t any person

So after no, cap strengthens the negative idea.

After cap, Catalan usually uses a singular noun, even when English would often use a plural.

So:

• cap tro = any thunder / any thunderclap
• not usually caps trons

This is normal Catalan grammar:

• cap llibre = no books / not a single book
• cap idea = no idea
• cap problema = no problems

English and Catalan behave differently here.

This is just the preposition a plus the article l' before a vowel sound:

• a + l'abril → a l'abril

It means:

• in April

Catalan often uses a with months in time expressions:

• al maig
• a l'abril
• al juny

So però a l'abril means but in April.

Sí que adds emphasis or contrast. It is like saying:

• did
• actually did
• did indeed

So:

• però a l'abril sí que hi va haver una tempesta

means something like:

• but in April there was a storm
• but in April there really was a storm
• but in April there actually was a storm

Here it contrasts with the first part:

• In May, we didn’t hear any thunder
• but in April, there definitely was a storm

So sí que is not just yes; it is an emphatic contrast marker.

In this sentence, hi ha / hi havia / hi va haver is the Catalan existential structure meaning there is / there was / there were.

So:

• hi ha una tempesta = there is a storm
• hi havia una tempesta = there was a storm
• hi va haver una tempesta = there was a storm

The hi is an essential part of this structure.

So hi va haver una tempesta does not mean it had a storm. It means there was a storm.

Catalan normally uses the indefinite article with a singular countable noun here, just as English does:

• una tempesta = a storm

This is completely regular:

• hi va haver un accident = there was an accident
• hi ha un problema = there is a problem

So una tempesta is simply the normal way to say a storm.

This follows a very natural Catalan order:

• sí que = emphatic contrast
• hi va haver = there was
• una tempesta = a storm

So the structure is:

[contrast/emphasis] + [existential verb] + [thing that existed]

Catalan word order is often a bit more fixed than English in this kind of sentence.
Putting sí que before the verb phrase is the normal way to emphasize the contrast.

Almost, but it helps to know the nuance.

Tro can mean:

• thunder
• a thunderclap
• the sound of thunder

In this sentence, cap tro means any thunder or any thunderclap.

So the idea is not about lightning in general, but specifically the sound associated with a storm.

Però is the normal Catalan word for but.

So:

• ..., però ... = ..., but ...

It links two contrasting ideas:

• last May, no thunder
• but in April, there was a storm

It is the most straightforward and common choice here.

Yes, but the meaning would be slightly different in tone or aspect.

• hi va haver una tempesta = there was a storm
  This presents the storm as a completed event.
• hi havia una tempesta = there was a storm / there used to be a storm / there was a storm going on
  This can sound more descriptive or ongoing, depending on context.

In this sentence, hi va haver fits well because it refers to a specific event in April.

Yes, but the tone is different.

• el maig passat is the normal, simple everyday expression: last May
• el passat mes de maig is more formal or elaborate: the past month of May

So for ordinary speech and writing, el maig passat is the most natural choice.