Ella está enamorada.

Because estar is the normal verb used with enamorado / enamorada.

In Spanish, estar enamorado/a means to be in love. Spanish treats this as a state or condition, so estar is used, not ser.

• Ella está enamorada = she is in love
• Ella es enamorada is not the normal way to say that

This does not mean the feeling is necessarily short-lived. A lot of emotional or physical states use estar, even if they last a long time.

Enamorada is functioning as an adjective here, meaning in love.

It comes from the verb enamorar / enamorarse, but in this sentence it behaves like an adjective after estar:

• está cansada = she is tired
• está contenta = she is happy
• está enamorada = she is in love

So you can think of enamorada as an adjective describing her emotional state.

Because it has to agree with ella, which is feminine singular.

Spanish adjectives often change form to match gender and number:

• enamorado = masculine singular
• enamorada = feminine singular
• enamorados = masculine plural / mixed plural
• enamoradas = feminine plural

So:

• Él está enamorado
• Ella está enamorada
• Ellos están enamorados
• Ellas están enamoradas

Very often, you can just say Está enamorada.

Spanish frequently leaves out subject pronouns when the subject is already clear from context. So both are possible:

• Ella está enamorada
• Está enamorada

Using ella can add:

• emphasis
• contrast
• clarity, if it is not obvious who you mean

For example:

• Él no, pero ella está enamorada = Not him, but she is in love.

Use de:

• Ella está enamorada de Juan
• Ella está enamorada de él
• Está enamorada de su profesor

So the pattern is:

estar enamorado/a de + person/thing

Not con in this structure.

The accent mark shows that the stress falls on the last syllable: es-TÁ.

It also helps distinguish the verb form está from esta.

• está = is
• esta = this (feminine)

So the accent is important in writing.

Not exactly.

• estar enamorada = to be in love (the state)
• enamorarse = to fall in love (the change or process)

Example:

• Se enamoró de Juan = She fell in love with Juan.
• Ahora está enamorada de Juan = Now she is in love with Juan.

So one describes becoming in love, and the other describes already being in that state.

Usually, yes. In most normal contexts, estar enamorada means to be romantically in love.

But Spanish can also use it more loosely or poetically for strong affection:

• Está enamorada de Madrid = She’s in love with Madrid.

That can mean she absolutely loves the city, not necessarily in a literal romantic sense. Still, the default meaning is usually romantic unless the context shows otherwise.