Ela mostrou interesse.

Mostrou comes from the infinitive mostrar, meaning to show.

In this sentence, mostrou is:

• 3rd person singular
• in the pretérito perfeito simples in Portuguese
• usually translated as the simple past: she showed

So Ela mostrou interesse describes a completed action in the past.

Yes, you can often leave it out.

Portuguese usually allows the subject pronoun to be omitted when it is clear from the context or from the verb form. So:

• Ela mostrou interesse.
• Mostrou interesse.

Both can be correct.

Including ela can add:

• clarity
• emphasis
• contrast, for example if you are distinguishing her from someone else

No. The verb does not change for gender here.

Mostrou is the same with:

• ele mostrou
• ela mostrou
• você mostrou

So the feminine meaning comes from ela, not from the verb form.

It is a noun. It means interest.

That is why the sentence is literally structured like:

• She showed interest

If you want to say interested, you need the adjective:

• interessado for masculine
• interessada for feminine

For example:

• Ela estava interessada. = She was interested.

Because mostrar interesse is a very common and natural expression in Portuguese.

With abstract nouns like interesse, Portuguese often does not need an article when speaking in a general sense.

So:

• mostrou interesse = showed interest

If you add an article or determiner, the meaning becomes more specific:

• mostrou um interesse especial = showed a particular/special interest
• mostrou muito interesse = showed a lot of interest

Yes, it is a very common collocation.

Portuguese speakers often use mostrar interesse to mean:

• to show interest
• to seem engaged
• to demonstrate curiosity or attraction toward something

It is natural in everyday speech and in more formal language too.

They are close, but not identical.

• Ela mostrou interesse focuses on an action or outward sign. She gave some indication that she was interested.
• Ela estava interessada focuses on her state or feeling. She was interested.

So:

• someone can be interested without clearly showing it
• someone can show interest in a conversation or situation, even briefly

Yes. Very often, you do.

Two common patterns are:

• interesse em
  • noun or infinitive
• interesse por
  • noun

Examples:

• Ela mostrou interesse em aprender português.
• Ela mostrou interesse em trabalhar connosco.
• Ela mostrou interesse por arte portuguesa.

Both em and por are common, and the choice depends a bit on what follows and on the exact meaning.

Yes, it can, but only if the context suggests that.

By itself, interesse is neutral. It could mean interest in:

• a person
• a topic
• a job
• an offer
• an idea

So Ela mostrou interesse does not automatically mean romantic interest. Context decides.

In European Portuguese, mostrou is pronounced roughly like mush-TROH.

A few useful points:

• the first o is usually reduced, sounding closer to u
• the s before t sounds like sh in Portugal
• the stress falls on the last syllable: -trou

So it does not sound like a fully pronounced mos-tro-u.

The normal order is:

• Ela mostrou interesse.

You can also say:

• Mostrou interesse.

That is very natural if the subject is already understood.

More unusual orders are possible for emphasis, but they are marked and less neutral. For a learner, the safest pattern is:

• subject + verb + object

The infinitive is mostrar.

Useful related forms:

• eu mostrei = I showed
• tu mostraste = you showed
• ele/ela/você mostrou = he/she/you showed
• nós mostrámos = we showed
• eles/elas/vocês mostraram = they/you showed

So this sentence is built from a very common verb, and it is worth learning as a full pattern.