Проект длился полгода.

Длился means lasted.

The base verb is длиться, which means to last, to continue for some time, or to go on.

So:

• Проект = project
• длился = lasted
• полгода = half a year / six months

So the whole sentence means The project lasted half a year.

Because the past tense in Russian agrees with the subject in gender and number.

The subject here is проект, which is:

• singular
• masculine

So the past-tense form must be masculine singular:

• длился = masculine
• длилась = feminine
• длилось = neuter
• длились = plural

Compare:

• Проект длился полгода.
• Программа длилась полгода.
• Задание длилось полгода.
• Переговоры длились полгода.

The dictionary form is длиться. The -ся is part of the verb.

In this verb, -ся does not mean that the subject is doing something to itself in a literal reflexive way. It is simply how this verb is formed. Russian has many verbs like this.

So:

• длиться = to last
• past masculine = длился

You should learn длиться as a complete verb, not as длить + ся in the English sense.

Because полгода is written as one word.

In Russian, пол- often combines with a noun to mean half of something. In this expression, the standard spelling is:

• полгода = half a year

This is a very common fixed expression.

So:

• полгода = half a year
• полчаса = half an hour
• полдня = half a day

A learner should usually memorize полгода as a single word.

Because after пол-, the noun is typically in the form historically based on the genitive singular.

So:

• год = year
• полгода = half a year

This is just the normal form you need here. It is best learned as a set expression:

• полгода
• полмесяца
• полчаса

You do not say полгод.

Usually yes in meaning, but not always in tone.

• полгода = half a year
• шесть месяцев = six months

Both can often be translated the same way.

But полгода is usually more natural and conversational when you mean a rough span of six months.
Шесть месяцев can sound a bit more precise, formal, or countable.

So both are possible:

• Проект длился полгода.
• Проект длился шесть месяцев.

The first one is very natural.

Because Russian often expresses duration with just the time expression, without a preposition.

So:

• Проект длился полгода. = The project lasted half a year.
• Я ждал час. = I waited for an hour.
• Он работал там три года. = He worked there for three years.

If you add за, the meaning changes.

• за полгода usually means within half a year or over the course of half a year, not simply for half a year

So Проект длился полгода is correct for duration.
Проект длился за полгода is not the right way to say this.

For a learner, the most useful answer is: treat полгода here as the normal duration expression meaning half a year.

More technically, полгода comes from пол- plus a noun form based on the genitive singular (года). But as a whole expression, it is commonly used in sentences like this without needing to analyze it every time.

So in practice, learn the pattern:

• длился час
• длился день
• длился месяц
• длился год
• длился полгода

The main thing is that this is the standard natural form.

Yes. Проект продолжался полгода is also correct and natural.

The difference is small:

• длился focuses on the duration: lasted
• продолжался focuses a bit more on the process continuing: continued / went on

In many contexts, both work well.

• Проект длился полгода. = The project lasted half a year.
• Проект продолжался полгода. = The project continued for half a year.

Yes. Russian word order is flexible.

The neutral order here is:

• Проект длился полгода.

But you could also say:

• Полгода длился проект.

That version puts more emphasis on полгода.

So the basic meaning stays the same, but the focus changes.

A helpful pronunciation guide is:

• Прое́кт дли́лся полго́да.

Stress:

• проЕкт
• длИлся
• полгОда

A rough English-style approximation would be:

• pra-YEKT DLEEL-sya pol-GO-da

But it is better to listen to native audio if possible, especially for:

• the consonant cluster in проект
• the лс sequence in длился
• the unstressed vowels, which are reduced in real speech

Not in this meaning. That sounds unnatural.

Russian normally uses a verb like:

• длиться = to last
• продолжаться = to continue

So for duration, say:

• Проект длился полгода.
• Проект продолжался полгода.

Using быть here does not express lasting the way English was sometimes can in loose speech.