A l'abril acostumem a veure núvols sobre la muntanya, però al maig gairebé sempre fa sol.

Both mean in April / in May, but the form changes because of the noun that follows.

• a + el abril becomes a l'abril
• a + el maig becomes al maig

So:

• a l'abril = in April
• al maig = in May

This happens because abril begins with a vowel, so the article el contracts to l'. With maig, which begins with a consonant, a + el becomes al.

This is very common in Catalan:

• a l'estiu
• al gener
• a l'agost

In Catalan, months often appear with the definite article, especially after prepositions like a.

So Catalan commonly says:

• a l'abril
• al maig
• al juny

This is different from English, where we just say in April or in May without an article.

You do not need to translate the article literally. It is just part of the normal Catalan structure.

Acostumem a veure means something like we usually tend to see or we are used to seeing.

It adds the idea of a habit or a typical pattern.

Compare:

• veiem núvols = we see clouds
• acostumem a veure núvols = we usually see clouds / we tend to see clouds

So acostumar helps express regularity or habit, not just a simple fact.

Because the verb acostumar is followed by a + infinitive when it means to usually do something.

Pattern:

• acostumar a + infinitive

Examples:

• Acostumo a llegir a la nit = I usually read at night
• Acostumem a veure núvols = We usually see clouds

So the a is required by the verb pattern.

Catalan often leaves out subject pronouns because the verb ending already tells you who the subject is.

Here:

• acostumem = we usually
• the ending -em shows it is nosaltres = we

So:

• Acostumem a veure... already means
• We usually see...

You could say Nosaltres acostumem a veure..., but it is usually unnecessary unless you want emphasis or contrast.

Acostumem is the 1st person plural present tense of acostumar.

That means:

• infinitive: acostumar
• present: acostumo, acostumes, acostuma, acostumem, acostumeu, acostumen

In this sentence, acostumem means we usually / we tend to.

Because Catalan often omits the article when talking about things in a general, non-specific way, especially in plural.

Here, núvols means clouds in general, not specific clouds already known to the listener.

So:

• veure núvols = to see clouds
• veure els núvols = to see the clouds specifically

In this sentence, the general meaning is the natural one.

Sobre means over / above.

So núvols sobre la muntanya means the clouds are over the mountain.

This does not necessarily mean they are touching it or sitting on top of it. It just places them above it.

Learners sometimes compare:

• sobre la muntanya = over the mountain
• damunt de la muntanya = on top of / above the mountain

In many contexts, sobre works well for clouds because they are hovering above something.

Gairebé sempre means almost always.

It comes before the verb phrase fa sol because it modifies how often that weather happens.

So:

• al maig gairebé sempre fa sol = in May it is almost always sunny

This position is very natural in Catalan.

You can think of the structure as:

• time
  • frequency expression
    • verb phrase

Here:

• al maig = time
• gairebé sempre = frequency
• fa sol = verb phrase

Catalan uses fer in many weather expressions.

So:

• fa sol = it is sunny
• literally, something like it makes sun, but you should learn it as a fixed weather expression

Other common examples:

• fa fred = it is cold
• fa calor = it is hot
• fa vent = it is windy
• fa bon temps = the weather is nice

This is one of those places where Catalan and English organize weather differently.

Because fa sol is an impersonal weather expression.

The verb is always in the 3rd person singular:

• fa sol
• fa fred
• fa calor

There is no real subject like the sun doing the action in the grammar of the sentence. It works a bit like English it is sunny, where it does not refer to a specific thing.

In standard Catalan, a l'abril and al maig are the normal choices in this kind of sentence.

Using en with months is not the standard pattern here.

So for a learner, it is best to use:

• a l'abril
• al maig
• al juny

That will sound natural and correct.

Però means but.

It introduces a contrast between the two parts:

• in April we usually see clouds over the mountain
• but in May it is almost always sunny

So però connects two ideas that contrast with each other. It is used just like English but.

Not in the same structure.

After acostumem, you need an infinitive:

• acostumem a veure ✅

You would not say:

• acostumem veiem ❌
• acostumem a veiem ❌

If you want to use veiem, you need a different sentence:

• A l'abril veiem núvols sobre la muntanya = In April we see clouds over the mountain

That is grammatical, but it loses the specific idea of habit or tendency carried by acostumem a veure.

Yes. It is very useful to learn fa sol as a chunk.

Catalan weather expressions often work best when memorized as whole phrases:

• fa sol
• fa fred
• fa calor
• fa vent
• plou
• neva

That will help you sound more natural than trying to build them word by word from English.