Demà volem fer una excursió al bosc si no plou.

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

• volem = we want
• The ending -em tells you it is we

So Demà volem... is completely natural. You could say Demà nosaltres volem..., but nosaltres would usually add emphasis or contrast, like we as opposed to someone else.

Yes. After voler (to want), Catalan normally uses an infinitive directly:

• volem fer = we want to do / we want to make
• vull anar = I want to go
• volen menjar = they want to eat

There is no extra word like to in English, and no preposition is needed here.

This is a normal Catalan expression. Literally, fer often means to do or to make, but in many everyday expressions it works where English uses a different verb.

So fer una excursió is a standard way to say to go on an excursion / to take a trip / to go for an outing.

Catalan often uses fer in set expressions, so it is best to learn fer una excursió as a chunk.

Because in this sentence it refers to one specific outing, so Catalan uses the indefinite article:

• una excursió = an excursion / a trip

Leaving out the article would sound unnatural here. Catalan generally uses articles with countable singular nouns in cases like this, just as English usually does.

Because al is the normal contraction of a + el:

• a = to
• el = the
• al = to the

So:

• al bosc = to the forest / woods

This contraction is required in normal Catalan.

Because Catalan does not use a dummy subject like English it with weather verbs.

• plou = it is raining / it rains
• nevava = it was snowing
• fa sol = it is sunny

English needs it in if it rains, but Catalan just says si plou.

After si meaning if, Catalan normally uses the present indicative to talk about a real future possibility.

So:

• si no plou = if it doesn’t rain
• not normally si no plourà

This is similar to English, which also says if it doesn’t rain, not if it won’t rain, in ordinary conditional sentences.

No. They are different.

• si no = if not / if ... not
• sinó = but rather / except

In this sentence, it must be si no plou because it means if it doesn’t rain.

Compare:

• Anirem si no plou. = We’ll go if it doesn’t rain.
• No anem al bosc, sinó a la platja. = We’re not going to the forest, but rather to the beach.

This is a very common thing for learners to confuse.

The word order can change. Catalan is fairly flexible, especially with time expressions.

These are all possible, with slightly different emphasis:

• Demà volem fer una excursió al bosc si no plou.
• Volem fer una excursió al bosc demà si no plou.
• Si no plou, demà volem fer una excursió al bosc.

Putting Demà first is a natural way to set the time right away.

They show where the stress falls.

• demà → stress on the last syllable: de-MÀ
• excursió → stress on the last syllable: ex-cur-si-Ó

In Catalan, written accents are important because they often tell you pronunciation and sometimes distinguish words that would otherwise look similar.

Yes. Both are possible, but they are slightly different expressions.

• fer una excursió = to do/go on an excursion
• anar d’excursió = to go on an outing / to go hiking / to go on a trip

Both are common, and in many contexts they are close in meaning. In your sentence, fer una excursió is perfectly natural.