Si el saco de dormir sigue mojado, no podré usarlo esta noche.

Si means if, so it introduces a condition: if the sleeping bag is still wet...

Cuando means when, which would suggest the speaker expects it to happen rather than presenting it as a possibility. In this sentence, the idea is conditional, so si is the correct choice.

• Si el saco de dormir sigue mojado... = If the sleeping bag is still wet...
• Cuando el saco de dormir siga mojado... would mean something more like When the sleeping bag is still wet..., which sounds different and less natural here.

Both are possible, but they mean slightly different things.

• está mojado = it is wet
• sigue mojado = it is still wet / it remains wet

Here, sigue mojado adds the idea that the sleeping bag was already wet before and has not dried yet. That matches the meaning of still in English.

So:

• Si el saco de dormir está mojado... = If the sleeping bag is wet...
• Si el saco de dormir sigue mojado... = If the sleeping bag is still wet...

Mojado has to agree with el saco, which is masculine singular.

Agreement works like this:

• el saco mojado = masculine singular
• la chaqueta mojada = feminine singular
• los sacos mojados = masculine plural
• las botas mojadas = feminine plural

Even though de dormir follows saco, the main noun is still saco, so the adjective agrees with that.

Literally, de dormir means for sleeping or to sleep.

So saco de dormir is literally something like sleeping bag or bag for sleeping. This is a very common Spanish structure:

• gafas de sol = sunglasses
• cepillo de dientes = toothbrush
• mesa de comedor = dining table

So de + infinitive/noun often describes the function of something.

Podré is the future tense of poder: I will be able to.

Since the sentence refers to esta noche and the consequence will happen later, Spanish naturally uses the future:

• no podré usarlo esta noche = I won’t be able to use it tonight

Compare:

• no puedo usarlo = I can’t use it (right now / in general)
• no podré usarlo = I won’t be able to use it
• no podría usarlo = I wouldn’t be able to use it / I couldn’t use it

Here, podré fits the idea best.

In Spanish, for real or likely conditions, the normal pattern is:

• si + present, future
• Si llueve, me quedaré en casa.
• Si tengo tiempo, te llamaré.

So this sentence follows that pattern:

• Si el saco de dormir sigue mojado, no podré usarlo esta noche.

English also often does this:

• If it rains, I’ll stay home.

So even though the result is in the future, the verb after si is usually present, not future.

No, that would normally be incorrect in standard Spanish for this kind of conditional sentence.

After si, when you are talking about a real possible future condition, Spanish uses the present, not the future:

• Correct: Si el saco de dormir sigue mojado...
• Incorrect here: Si el saco de dormir seguirá mojado...

This is one of the most important patterns to remember:

• si + present + future

Because -lo means it, and it refers back to el saco de dormir.

• usar = to use
• usarlo = to use it

Spanish often attaches object pronouns to the end of an infinitive:

• quiero verlo = I want to see it/him
• necesito comprarlo = I need to buy it
• no podré usarlo = I won’t be able to use it

You could also think of it as:

• No lo podré usar which is also correct.

Both mean the same thing:

• no podré usarlo
• no lo podré usar

Lo is the direct object pronoun for a masculine singular noun, and it refers to el saco de dormir.

Since saco is masculine singular, the pronoun is lo:

• el saco → lo
• la tienda → la
• los sacos → los
• las botas → las

So:

• usar el saco de dormir = to use the sleeping bag
• usarlo = to use it

Spanish often uses time expressions directly, without a preposition, where English uses one.

• esta noche = tonight
• hoy = today
• mañana = tomorrow
• este verano = this summer

So:

• no podré usarlo esta noche = I won’t be able to use it tonight

You do not need en here.

They are very similar, and in many contexts they can mean the same thing.

• esta noche = tonight
• hoy por la noche = tonight / today at night / this evening-tonight

Esta noche is shorter and very natural here.
Hoy por la noche is a bit more explicit.

So both could work:

• No podré usarlo esta noche.
• No podré usarlo hoy por la noche.

The first one is the most natural and direct in this sentence.

Yes. That is completely correct.

Spanish allows both orders:

• Si el saco de dormir sigue mojado, no podré usarlo esta noche.
• No podré usarlo esta noche si el saco de dormir sigue mojado.

The meaning stays the same. The version with si first often sounds a bit more like the speaker is emphasizing the condition first.

Yes, but they sound a bit different.

• sigue mojado = still wet / remains wet
  This is the most natural everyday option.
• continúa mojado = continues to be wet
  Correct, but a bit more formal or written.
• permanece mojado = remains wet
  Also correct, but less common in casual speech.

So for normal conversation, sigue mojado is usually the best choice.

Because saco by itself can mean different things depending on context, such as a sack, bag, or even a jacket in some varieties. Saco de dormir makes it completely clear that we mean a sleeping bag.

Spanish often uses longer noun phrases where English uses one compact noun:

• saco de dormir = sleeping bag
• bolsa de basura = garbage bag
• máquina de lavar = washing machine

So the full expression avoids ambiguity.