Learn Hardcore Filipino: Basa ang payong kapag umuulan. - The umbrella is wet when it is raining.

Here it’s basâ meaning wet (an adjective). It’s a homograph with bása (the root for read). Filipino usually drops accent marks, so stress distinguishes them:

Basâ ang payong. The umbrella is wet. (final-syllable stress with a slight glottal stop at the end)
Nagbabása ako. I am reading. (stress on the second syllable of the root) Tip: You’ll sometimes see the wet sense written as basâ and the read sense as bása in learning materials.

Why is there no word for is?

In Filipino, adjectives can serve directly as predicates. So Basâ ang payong literally means Wet the umbrella. There’s no separate copula like is. You can also invert to a more formal written style: Ang payong ay basâ.

What does ang do before payong? Is it just the?

Ang marks the topic/subject of the clause. It often translates as the, but its job is grammatical, not just definiteness. In general statements, ang + singular noun can refer to a whole class (generic):

Basâ ang payong kapag umuulan. Umbrellas are wet when it rains. (generic) If you want to be explicitly plural, add mga:
Basâ ang mga payong kapag umuulan.

Should I use mga to say umbrellas (plural) in this sentence?

Not required. Generic statements commonly use the singular with ang to refer to a class. Both are acceptable:

Generic: Basâ ang payong kapag umuulan.
Explicit plural: Basâ ang mga payong kapag umuulan. The meaning is the same; the second just highlights plurality.

What’s the difference between kapag, kung, and pag?

Kapag = when/whenever (real/habitual time reference). It’s the safest choice for when.
Pag = informal/short form of kapag in speech and casual writing.
Kung = if (conditional, hypothetical). In casual speech some people use kung where kapag would be more precise, but learners should keep the distinction. Here, kapag umuulan means whenever it rains/when it rains.

Why is it umuulan and not umulan or uulan after kapag?

Umuulan (imperfective) = it is raining / it rains (habitually). Good for general truths: Kapag umuulan…
Umulan (completed) = it rained. With time words it can mean when it rained (a specific past event): Noong umulan, basâ ang payong.
Uulan (contemplated) = it will rain. For future: Kapag uulan mamaya, mababasa ang payong. In your sentence, we’re stating a habitual truth, so umuulan fits best.

How is umuulan formed from ulan?

Root: ulan (rain)

Completed: umulan (it rained)
Imperfective/ongoing or habitual: umuulan (it’s raining / it rains)
Contemplated/future: uulan (it will rain) Mechanics: the intransitive infix -um- attaches to vowel-initial roots as a prefix (umulan), and the imperfective duplicates the first vowel/syllable (hence umuulan).

Is basa an adjective or a verb here? Could I say Nababasa ang payong?

Here basâ is an adjective (stative): the umbrella is wet.
Nababasa is a verb (gets wet/is getting wet). Use it if you want the dynamic sense:

Stative: Basâ ang payong kapag umuulan. The umbrella is wet when it rains. (general state)
Dynamic: Nababasa ang payong kapag umuulan. The umbrella gets wet when it rains. (process/change)

How do I say the umbrella got wet when it rained (a specific event)?

Use the completed aspect for both:

Nabasa ang payong noong umulan. The umbrella got wet when it rained.

How do I say wet umbrella (as a noun phrase) versus the umbrella is wet (a sentence)?

Attributive (noun phrase): Basang payong = wet umbrella.
This uses the linker -ng after a vowel: basa + ng → basang.
Predicative (clause): Basâ ang payong. = The umbrella is wet.

Can I also say Ang payong ay basâ? What’s the difference in word order?

Yes. Basâ ang payong (predicate-initial) is the default, conversational order. Ang payong ay basâ uses the topic-fronting marker ay and is more formal or written. Both are correct.

How do I pronounce the sentence? Where is the stress?

Natural stress is:

Basâ ang payóng kapag umuulán. Guides:
Basâ: stress on the last syllable, with a slight glottal stop at the end.
Payóng: stress on the second syllable.
Umuulán: stress on the last syllable. Accent marks are optional in real life; they’re shown here to help you hear the stress.

How do I negate it?

Use hindi:

Simple negation now: Hindi basâ ang payong. The umbrella is not wet.
Contrasting condition: Kapag hindi umuulan, tuyô ang payong. When it isn’t raining, the umbrella is dry.
Note: tuyô = dry.

Do I need an it subject for rains, like in English it rains?

No. Umuulan by itself means it’s raining. Filipino doesn’t use a dummy subject like English it in weather expressions.

Are there related words I can use for rain or wetness?

Yes:

Maulán = rainy (as in rainy weather/day). Example: Kapag maulán, basâ ang payong.
Strong rain verbs: bumubuhos (pouring), malakas ang ulan (the rain is strong).
Wetness intensifier: basang-basâ (soaking wet). Example: Basang-basâ ang payong pagkatapos ng bagyo.