Si no plou, podem fer l'excursió al bosc.

In Catalan, no normally goes directly before the verb, so no plou means it doesn’t rain.

• si = if
• no plou = it doesn’t rain

So si no plou is the natural way to say if it doesn’t rain.

Plou is the 3rd person singular present indicative of ploure (to rain).

Weather verbs in Catalan are usually used in the 3rd person singular:

• plou = it rains / it is raining
• nevava = it was snowing
• ha plogut = it has rained

Like English it rains, Catalan uses this verb without naming a real subject.

Catalan does not use a dummy subject like English it in weather expressions.

So:

• English: it rains
• Catalan: plou

The verb appears by itself. This is completely normal with weather verbs such as:

• plou = it’s raining
• neva = it’s snowing
• fa sol = it’s sunny

Podem fer means we can do / we can go ahead with. It expresses possibility or permission, not just a future action.

• fem l’excursió = we do the excursion / we’re doing the excursion
• farem l’excursió = we will do the excursion
• podem fer l’excursió = we can do the excursion

So the sentence means that the trip is possible if it doesn’t rain.

Also, Catalan often uses the present tense for future meaning when the context is clear, so podem fer can sound very natural here.

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

• podem = we can

So nosaltres podem is possible, but usually unnecessary unless you want emphasis or contrast.

For example:

• Podem fer l’excursió = We can do the excursion
• Nosaltres podem fer l’excursió, però ells no = We can do the excursion, but they can’t

Because the feminine singular article la becomes l’ before a vowel sound.

• la casa
• la muntanya
• l’excursió
• l’escola

So la excursió is not correct; it contracts to l’excursió.

Literally, fer means to do / to make, but Catalan often uses fer in expressions where English might prefer a different verb.

So fer una excursió means to go on a hike/trip/excursion or to do the excursion.

This is a common Catalan pattern:

• fer una pregunta = ask a question
• fer una foto = take a photo
• fer una excursió = go on an excursion / take a trip

Al is the contraction of a + el.

• a = to / at
• el bosc = the forest
• al bosc = to the forest / in the forest, depending on context

This kind of contraction is very common:

• a + el = al
• de + el = del

So al bosc is the correct form.

It can suggest either one depending on context, but here it most naturally means something like an excursion to the forest or in the forest.

With fer una excursió al bosc, Catalan is expressing the destination or setting of the excursion. English can translate this in different natural ways:

• go on an excursion to the forest
• go on a trip to the woods
• go for a hike in the forest

So the exact English preposition may vary, but the Catalan expression is perfectly normal.

Catalan often uses the present tense for future situations when the meaning is clear from context.

So:

• Si no plou, podem fer l’excursió al bosc
  means something like If it doesn’t rain, we can go on the excursion to the forest

This is very natural in Catalan. English does something similar in the if-clause:

• English: If it doesn’t rain, we’ll go
• not usually If it won’t rain

Catalan also avoids future after si in this kind of real condition.

The comma is standard and helpful when the conditional clause comes first.

• Si no plou, podem fer l’excursió al bosc.

It separates the if-clause from the main clause, just as in English. In informal writing, people may sometimes omit it, but using it is a good habit.