Pupunta tayo sa kampo basta hindi umuulan.

Because tayo is inclusive we: it includes the person being spoken to.
So pupunta tayo means we will go, including you.

By contrast, kami is exclusive we: it excludes the listener.
So pupunta kami would mean we will go, but not you.

Sa is a marker often used for location or direction.
With a motion verb like pupunta, it usually means to.

So sa kampo here means to the camp.
With other verbs, sa can also mean at, in, or on, depending on context.

Because sa is the correct marker for a destination/location phrase here.
Ang usually marks the topic/subject of the sentence, so it would not fit naturally in this slot.

So after pupunta, you normally say sa kampo, not ang kampo.

Filipino does not use articles the same way English does. There is no exact one-word equivalent that always matches English the.

In this sentence, sa kampo can naturally mean to the camp. The specificity is often understood from context rather than from a separate article.

Here, basta means as long as, provided that, or so long as.

So it introduces a condition:

• basta hindi umuulan = as long as it isn’t raining

Be aware that basta can mean other things in other contexts, such as a casual just or anyway, but here it is clearly conditional.

Yes, but the nuance changes a little.

• kung hindi umuulan = if it isn’t raining
• basta hindi umuulan = as long as / provided it isn’t raining

Basta sounds a bit stronger, as if this is the requirement that must be met.

Hindi is the normal negator for verbs, adjectives, and whole clauses. Since umuulan is a verb, hindi umuulan is correct.

• hindi umuulan = it isn’t raining

Yes, ’di is a common shorter, more casual form of hindi in speech and informal writing.

Wala is different: it usually means there is none, there isn’t any, or absent/nonexistent. It is not the normal way to negate umuulan.

These are different forms of the verb based on ulan:

• umuulan = is raining / raining
• umulan = rained or sometimes to rain, depending on context
• uulan = will rain

In this sentence, the speaker is talking about the condition at the time of going: if it is not raining, then they will go. That is why hindi umuulan is the natural choice.

Because Filipino often uses the imperfective form in conditions like this when describing a situation that may be true at the relevant time.

English does something similar:

• We’ll go if it isn’t raining not usually
• We’ll go if it won’t be raining

So hindi umuulan is perfectly natural even though pupunta refers to a future action.

There isn’t one. Filipino does not need a dummy subject like English it in weather expressions.

So umuulan by itself already means it is raining or it rains. The subject is simply not expressed because Filipino does not require it here.

Yes. You can also say:

Basta hindi umuulan, pupunta tayo sa kampo.

That is very natural too. The meaning stays basically the same, but putting the condition first gives it a bit more emphasis.