Esa diadema azul combina bien con tu vestido.

Spanish has three common demonstratives:

• esta = this
• esa = that
• aquella = that over there / that more distant one

Here, esa diadema means that headband.
A native speaker might choose esa because the item is closer to the listener, not especially close to the speaker, or simply because it has already been identified in the conversation.

So:

• esta diadema = this headband
• esa diadema = that headband
• aquella diadema = that headband over there

Also, esa has to agree with diadema, which is feminine singular.

In everyday Spanish from Spain, diadema usually means a headband or hairband worn on the head.

In more formal or literary contexts, it can also mean a diadem or tiara, but in a sentence about matching clothes, most learners should understand it as headband.

Because the subject is esa diadema azul, which is third-person singular.

So the verb combinar is conjugated as:

• yo combino
• tú combinas
• él / ella / usted combina
• ellos / ellas / ustedes combinan

In this sentence, the headband is doing the action of matching, so Spanish uses combina:

• Esa diadema azul combina...

It is not based on tu vestido. The verb agrees with the subject, not with the thing after con.

In Spanish, descriptive adjectives usually come after the noun, especially colors.

So:

• diadema azul = blue headband
• vestido rojo = red dress
• casa blanca = white house

That is the normal, neutral order in Spanish.
Putting the adjective before the noun is much less common and usually adds emphasis, style, or a literary tone.

Because azul is one of many adjectives that do not change for gender in the singular.

So you get:

• el vestido azul
• la diadema azul

Both masculine and feminine singular use azul.

It only changes in the plural:

• los vestidos azules
• las diademas azules

So the important agreement here is:

• singular azul
• plural azules

not masculine vs. feminine.

Not exactly.

Although combinar can sometimes mean to combine, in clothing and fashion it very often means:

• to match
• to go well with

So in this sentence, combina bien con means matches well with or goes well with.

That is a very common use in Spanish:

• Estos zapatos combinan con la chaqueta.
• El bolso no combina con la falda.

So learners should not always translate combinar as combine.

Because bien is an adverb, and it modifies the verb combina.

• combina bien = matches well

By contrast, bueno / buena are adjectives and describe nouns, not verbs.

Compare:

• Es una diadema buena. = It is a good headband.
• Combina bien. = It matches well.

So in this sentence, the idea is not that the headband is good, but that it matches well.

Because when you say what something matches with, Spanish normally uses combinar con.

So:

• combina con tu vestido = it matches with your dress

This is the standard pattern:

• La camisa combina con los pantalones.
• Ese bolso combina con tus zapatos.

Without con, the sentence would sound incomplete in this context, because the speaker wants to say what the headband matches with.

Because tu without an accent is the possessive adjective meaning your.

• tu vestido = your dress

With an accent, tú is the subject pronoun meaning you.

• Tú llevas ese vestido. = You are wearing that dress.

So:

• tu = your
• tú = you

That accent matters.

Because in Spanish, possessive adjectives usually replace the article.

So Spanish says:

• tu vestido
• mi casa
• su coche

not:

• el tu vestido
• la mi casa

So tu vestido is the normal structure for your dress.

Yes. If you were addressing someone formally in Spain, you would normally use su vestido instead of tu vestido.

So:

• tu vestido = your dress, informal
• su vestido = your dress, formal

Be aware that su can also mean his, her, or their, so context is important.

Yes. A few natural alternatives are:

• va bien con tu vestido = goes well with your dress
• queda bien con tu vestido = looks good with your dress

There is a slight difference in feel:

• combina bien con focuses on matching
• va bien con is a bit more general and conversational
• queda bien often focuses more on how it looks when worn

For example:

• Esa diadema azul combina bien con tu vestido.
• Esa diadema azul va bien con tu vestido.
• Esa diadema azul te queda bien con ese vestido.

All are possible, but combina bien con is a very clear, standard way to talk about colors and clothing matching.