Она была несчастной.

A simple breakdown is:

• Она = she
• была = was
  (the feminine singular past-tense form of быть, to be)
• несчастной = unhappy / unfortunate
  (a feminine singular form of несчастный)

So the whole sentence means She was unhappy or, depending on context, She was unfortunate.

Russian usually drops to be only in the present tense.

So:

• Она несчастна = She is unhappy
• Она была несчастной = She was unhappy
• Она будет несчастной = She will be unhappy

In the past and future, Russian normally uses forms of быть.

In the past tense, Russian verbs agree with the subject in gender and number.

Because она means she, the verb must be feminine singular:

• он был = he was
• она была = she was
• оно было = it was
• они были = they were

So была is used because the subject is feminine.

Because after была, Russian often uses the predicate adjective in the instrumental case.

The basic feminine form is:

• несчастная = unhappy (dictionary-style feminine form)

But in this sentence, after была, it becomes:

• несчастной

This is a very common pattern with past and future forms of быть.

So:

• Она была несчастной = correct, standard pattern

The dictionary form is несчастный.

That is the masculine singular nominative form, which is the standard form dictionaries use for adjectives.

Related forms are:

• несчастный — masculine
• несчастная — feminine
• несчастное — neuter
• несчастные — plural

In your sentence, the adjective appears as несчастной because it is agreeing with a feminine subject and is being used in the instrumental case.

Yes. Она была несчастна is also correct.

There is a difference in form:

• несчастной = long-form adjective, instrumental
• несчастна = short-form adjective

Both can mean She was unhappy.

Very roughly:

• Она была несчастной sounds neutral and descriptive.
• Она была несчастна can sound a bit more literary, formal, or focused on her state.

For a learner, it is good to recognize both.

It can mean either, depending on context.

Common possibilities:

• emotionally unhappy
• unfortunate, ill-fated, tragic

Without more context, She was unhappy is often the most natural interpretation. But Russian несчастный is broader than English unhappy.

By itself, the sentence does not fully decide that.

Она была несчастной can mean:

• she felt unhappy during some period
• she lived an unhappy life
• she was an unfortunate person

The exact nuance comes from context. The sentence alone just tells you that, at some point in the past, this description was true of her.

Yes. Russian word order is flexible.

The neutral order is:

• Она была несчастной.

But other orders are possible for emphasis, for example:

• Несчастной она была.
  This emphasizes unhappy.

The meaning stays similar, but the focus changes. For learners, Она была несчастной is the safest neutral version.

You can compare the three tenses:

• Она несчастна. = She is unhappy.
• Она была несчастной. = She was unhappy.
• Она будет несчастной. = She will be unhappy.

This is a useful pattern to remember:

• present: often no to be
• past/future: use forms of быть

A simple pronunciation guide is:

• Она — a-NA
• была — by-LA
• несчастной — nee-SHAST-noy

The main stresses are:

• она́
• была́
• несча́стной

So the full sentence is approximately:

a-NA by-LA nee-SHAST-noy