Нужна уплотнительная лента.

Because нужна agrees with лента, which is a feminine singular noun.

Russian changes this word according to the gender and number of the thing that is needed:

• нужен — masculine singular
• нужна — feminine singular
• нужно — neuter singular
• нужны — plural

So:

• нужна лента — a tape/band is needed
• нужен инструмент — a tool is needed
• нужно время — time is needed
• нужны детали — parts are needed

A major clue is the ending -а. Many Russian nouns ending in -а are feminine.

So лента is feminine, and that is why the predicate is нужна.

Other similar examples:

• нужна книга
• нужна дверь
• нужна прокладка

Russian often leaves that part out when it is obvious from context or when the speaker is making a general statement.

So Нужна уплотнительная лента can mean something like:

• A sealing tape is needed
• We need sealing tape
• I need sealing tape

If you want to make it explicit, you can add a dative pronoun:

• Мне нужна уплотнительная лента — I need sealing tape
• Нам нужна уплотнительная лента — We need sealing tape
• Вам нужна уплотнительная лента — You need sealing tape

They are in the nominative singular feminine.

In this kind of sentence, the thing needed is normally in the nominative:

• Нужна лента
• Нужен ключ
• Нужны перчатки

Here:

• лента — nominative singular feminine
• уплотнительная — nominative singular feminine, agreeing with лента

Because they do different jobs.

• уплотнительная is a regular full adjective modifying the noun лента
• нужна is the short form of нужный, used predicatively, meaning needed / necessary

So in the sentence:

• уплотнительная лента = sealing tape
• нужна = is needed

This short-form use is very common:

• Дверь закрыта — The door is closed
• Работа готова — The work is ready
• Помощь нужна — Help is needed

It literally means something like sealing tape, sealing strip, or gasket tape, depending on context.

The adjective comes from the verb уплотнять, meaning to make tighter / denser / to seal.

So уплотнительная лента is tape or strip material used to seal gaps, prevent leaks, or make a connection tighter.

The exact English translation depends on the situation:

• plumbing
• windows/doors
• construction
• machinery

In the present tense, Russian usually does not use a word for is / am / are.

So:

• Нужна уплотнительная лента literally looks like Needed sealing tape
• but naturally means A sealing tape is needed

This is normal in Russian:

• Это стол — This is a table
• Она дома — She is at home
• Лента нужна — The tape is needed

Yes. Russian word order is flexible, and changing it often changes focus rather than core meaning.

For example:

• Нужна уплотнительная лента — neutral; introduces what is needed
• Уплотнительная лента нужна — emphasizes that the sealing tape is what is needed
• Лента нужна — shorter, if the type of tape is already understood

So the original sentence is a very natural neutral way to say it.

The stress is:

• нужна́
• уплотни́тельная
• ле́нта

So: нужна́ уплотни́тельная ле́нта

A rough pronunciation guide:

• нужна ≈ noozh-NA
• уплотнительная ≈ oo-plut-NEE-tiĺ-na-ya
• лента ≈ LYEN-ta

The stress is important, especially in нужна́.

Yes — and in many situations that is the most natural version if you mean I need sealing tape.

Compare:

• Нужна уплотнительная лента — Sealing tape is needed / general statement
• Мне нужна уплотнительная лента — I need sealing tape
• Нам нужна уплотнительная лента — We need sealing tape

So the original sentence is correct, but it is slightly less personal unless the context already makes the subject clear.

Yes. Memorize this agreement pattern:

• нужен
  • masculine singular noun
    
    • Нужен ключ
• нужна
  • feminine singular noun
    
    • Нужна лента
• нужно
  • neuter singular noun
    
    • Нужно окно
• нужны
  • plural noun
    
    • Нужны материалы

This is one of the most useful patterns in everyday Russian.