Эта победа была важной.

Because победа is a feminine noun.

Russian demonstratives must agree with the noun they describe:

• этот = masculine
• эта = feminine
• это = neuter
• эти = plural

Since победа is feminine, you say эта победа.

A very common clue is the ending -а in the dictionary form. Most nouns ending in -а / -я are feminine.

So:

• победа = victory → feminine
• therefore words describing it also take feminine forms:
  • эта
  • была
  • важной

As always, there are some exceptions in Russian, but победа is a regular feminine noun.

Была is the feminine singular past tense form of быть (to be).

In the past tense, Russian verbs agree with gender and number:

• был = masculine
• была = feminine
• было = neuter
• были = plural

Since the subject is победа (a feminine noun), the verb is была:

• Эта победа была...

Because after была here, the adjective is being used as a predicate adjective, and Russian commonly puts that adjective in the instrumental case.

So:

• важная = nominative feminine singular
• важной = instrumental feminine singular

In Эта победа была важной, важной means important as the description assigned to the subject after was.

This is a very common pattern in Russian:

• Она была красивой. = She was beautiful.
• Книга была интересной. = The book was interesting.

So the key idea is:

• победа is the subject
• важной is the description after was
• that description often goes into the instrumental

It is an adjective: the adjective важный means important.

Its basic forms are:

• важный = masculine
• важная = feminine
• важное = neuter
• важные = plural

In your sentence, it appears in the feminine instrumental singular form:

• важной

That form matches победа in gender and number, and it also shows the case required by the construction.

• победа is nominative singular because it is the subject of the sentence.
• важной is instrumental singular because it is the predicate adjective after была.

So the structure is:

• Эта победа → subject, nominative
• была важной → was important, with the adjective in instrumental

This difference in case is one of the things that often feels unfamiliar to English speakers.

You may encounter forms like that in some contexts, but Эта победа была важной is the standard and safest choice here.

For learners, the best rule is:

• after past/future forms of быть, a noun or adjective used as the complement is very often instrumental

So if you want to say This victory was important, use:

• Эта победа была важной.

Russian has no articles.

So победа can mean:

• a victory
• the victory

The exact meaning depends on context.

In this sentence, эта already makes it specific:

• эта победа = this victory

So Russian does not need a separate word for the.

Russian usually does not use a present-tense form of to be in ordinary sentences.

So instead of a literal present-tense is, Russian often just says:

• Эта победа важна. = This victory is important.

A more formal option is:

• Эта победа является важной.

But in normal everyday Russian, Эта победа важна is much more natural than trying to insert a present-tense быть.

This is useful to compare with your sentence:

• Эта победа была важной. = This victory was important.
• Эта победа важна. = This victory is important.

The neutral word order is:

• Эта победа была важной.

That is the most straightforward way to say it.

Russian word order is more flexible than English, so other orders are possible for emphasis, for example:

• Важной была эта победа.
• Была важной эта победа.

But those sound more marked or literary. For a learner, the normal pattern to remember is:

• [demonstrative] + [noun] + [past form of быть] + [predicate adjective]

So:

• Эта победа была важной.

A helpful stress guide is:

• Э́та побе́да была́ ва́жной.

Approximate pronunciation:

• EH-ta pa-BYE-da buh-LAH VAZH-nay

A few notes:

• победа has stress on -бе-
• была has stress on the last syllable
• важной has stress on the first syllable

Listening and repeating is especially important with Russian vowel reduction, because unstressed vowels often sound weaker than they look in writing.

Both relate to important, but they are different forms used in different structures.

• важной = long-form adjective, feminine instrumental
• важна = short-form adjective, feminine singular

Examples:

• Эта победа была важной. = This victory was important.
• Эта победа важна. = This victory is important.

For learners, a useful shortcut is:

• after был / была / было / были, you will often see the instrumental form like важной
• in present-tense statements without to be, Russian often uses a short adjective like важна

Both are completely normal, but they appear in different grammar patterns.