La tormenta amenaza la costa.

Because both tormenta and costa are feminine singular nouns, so they take the feminine singular definite article la.

• la tormenta = the storm
• la costa = the coast

A learner will often notice that both nouns end in -a, which very often signals a feminine noun in Spanish, though not always.

Here, amenaza is a verb: the third-person singular present tense of amenazar.

So:

• amenazar = to threaten
• amenaza = threatens

That means the structure is:

• La tormenta = subject
• amenaza = verb
• la costa = direct object

There is also a noun amenaza meaning threat, but not in this sentence. If it were a noun, the sentence would need a different structure.

Because the subject is la tormenta, which is singular.

In Spanish, the verb changes to match the subject:

• la tormenta amenaza = the storm threatens
• las tormentas amenazan = the storms threaten

So the verb must be singular here.

Because la costa is a thing/place, not a person.

Spanish uses the personal a mainly before a specific human being or sometimes a pet/animal with personal value. For example:

• Veo a María = I see María

But with things or places, you normally do not use that a:

• La tormenta amenaza la costa
• not La tormenta amenaza a la costa

So la costa is simply the direct object without the personal a.

Because subject + verb + object is the most neutral and common word order in Spanish too.

This sentence follows that very basic pattern:

• La tormenta = subject
• amenaza = verb
• la costa = object

Spanish can sometimes change word order for emphasis or style, but this version is the most straightforward and natural.

Yes. That is a correct passive-style way to express the idea.

• La tormenta amenaza la costa = active
• La costa está amenazada por la tormenta = passive/result-focused

The first version is more direct and usually more natural in everyday Spanish. The second puts the focus on the coast instead of the storm.

Yes, that is also grammatical, but the meaning is slightly different in feel.

• La tormenta amenaza la costa = simple present; general/current fact
• La tormenta está amenazando la costa = the storm is threatening the coast right now, with more emphasis on the ongoing action

In Spanish, the simple present is often enough where English might use the present continuous.

In standard Peninsular Spanish, the z is pronounced like the th in think.

So amenaza is approximately:

• a-me-NA-tha

And the whole sentence is roughly:

• la tor-MEN-ta a-me-NA-tha la COS-ta

A few pronunciation notes:

• tormenta: stress on men
• amenaza: stress on na
• costa: stress on cos

In many Latin American accents, that z would sound like s instead.

Spanish usually does not need a subject pronoun when the subject is already stated.

English needs it threatens, but Spanish can simply say:

• La tormenta amenaza

Once the noun la tormenta is there, there is no need to add a separate word for it.

If the subject were already understood from context, Spanish could even just say:

• Amenaza la costa

But with no context, that would sound less clear than the full sentence.

Usually, yes. Tormenta is a general word for storm, often especially a storm with violent weather such as wind, rain, thunder, or rough seas.

Depending on context, Spanish can also use more specific words, such as:

• tempestad = tempest / violent storm
• tormenta eléctrica = thunderstorm

But tormenta is a very normal, common word here.

Spanish often uses articles more regularly than English does. In this sentence, la costa refers to the coast as the thing being threatened, so the definite article sounds natural.

Leaving it out would be ungrammatical here:

• La tormenta amenaza la costa = correct
• La tormenta amenaza costa = incorrect

So in this sentence, la is required.