Si vas a la muntanya amb la meva filla, no has d'oblidar la motxilla i les ulleres.

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

• vas = you go
• has = you have

So Si vas... already means If you go..., and no has d'oblidar... already means you mustn’t forget...

You could add tu for emphasis, but it is not normally necessary:

• Si tu vas a la muntanya... = If you go to the mountain/mountains... with extra emphasis on you

After si meaning if, Catalan normally uses the present indicative for a real possible situation.

So:

• Si vas a la muntanya... = If you go to the mountains...

Not:

• Si aniràs... in this kind of sentence

This is actually similar to English:

• If you go..., not If you will go...

So the structure is very natural in Catalan:

• Si + present, then the result clause

Anar a la muntanya literally looks like to go to the mountain, but in real use it often means something like:

• to go to the mountains
• to go up into the mountains
• to go mountain hiking / to the countryside in the mountains

Catalan often uses the singular here where English may prefer the mountains.

So a la muntanya is often an idiomatic place expression, not necessarily one single specific mountain.

In Catalan, possessives usually go with a definite article:

• la meva filla = my daughter
• el meu pare = my father
• les meves ulleres = my glasses

So the normal pattern is:

• article + possessive + noun

That is why you get:

• amb la meva filla = with my daughter

English does not use the here, but Catalan usually does.

That is the standard Catalan word order for possessives:

• la meva filla
• el meu llibre
• els nostres amics

So unlike English my daughter, Catalan usually says the my daughter if translated word-for-word, though of course that is not how it should be translated.

This is just the normal Catalan structure:

• article + possessive + noun

The expression haver de + infinitive means:

• to have to
• to must / must

So:

• has d'oblidar literally = you have to forget

But because the sentence is negative:

• no has d'oblidar la motxilla i les ulleres

the natural meaning is:

• you mustn’t forget the backpack and the glasses
• don’t forget the backpack and the glasses

A very common, more direct way to say this in Catalan would also be:

• No oblidis la motxilla i les ulleres.

So this sentence is using an obligation structure rather than the imperative.

The apostrophe shows elision.

The full form is:

• de oblidar

But Catalan normally contracts de before a vowel:

• d'oblidar

This is very common:

• d'anar
• d'estudiar
• d'obrir

So:

• has d'oblidar = you have to forget / you must not forget in context

Catalan uses definite articles more often than English.

So if the speaker means specific, known items, Catalan naturally says:

• la motxilla = the backpack
• les ulleres = the glasses

In English, we often leave this more flexible:

• Don’t forget your backpack and glasses
• Don’t forget the backpack and the glasses

Catalan often prefers the article when the items are understood from context.

Because ulleres means glasses, and like English, it is normally used in the plural.

• una ullera is not the usual everyday way to talk about a pair of glasses
• les ulleres = the glasses

So:

• la motxilla i les ulleres = the backpack and the glasses

This is similar to English glasses, scissors, etc., where the everyday word is plural.

Yes. That is a very common and very natural alternative.

Compare:

• No has d'oblidar la motxilla i les ulleres.
  = You mustn’t forget / Don’t forget the backpack and the glasses.

• No oblidis la motxilla i les ulleres.
  = Don’t forget the backpack and the glasses.

The second one is more directly like an instruction or reminder. The first one uses the haver de structure and sounds a bit more like stating an obligation.

The sentence begins with a conditional clause:

• Si vas a la muntanya amb la meva filla = If you go to the mountains with my daughter

Then comes the main clause:

• no has d'oblidar la motxilla i les ulleres = don’t forget the backpack and the glasses

Catalan commonly uses a comma after an introductory si clause, just as English often does:

• If you go..., don’t forget...

So the comma helps separate the two parts of the sentence clearly.