Во время учебной тревоги нельзя пользоваться лифтом.

Во время is the normal expression meaning during.

Russian sometimes uses во instead of в before a difficult consonant cluster to make pronunciation easier. Since время begins with вр-, во время sounds smoother than в время.

So this is both:

• a pronunciation convenience, and
• a fixed common phrase

Examples:

• во время урока — during the lesson
• во время дождя — during the rain

Because во время requires the genitive case.

The basic dictionary form is:

• учебная тревога — training alarm, drill alarm

After во время, it changes to genitive singular:

• учебной тревоги

Breakdown:

• учебная → учебной
• тревога → тревоги

So the whole phrase means during a drill / during a practice alarm.

Literally, it means something like training alarm or practice emergency alarm.

In natural English, depending on context, it could be:

• a drill
• a fire drill
• an evacuation drill
• a practice alarm

The adjective учебный / учебная means training, instructional, or for practice. It tells you the alarm is not necessarily a real emergency, but a planned exercise.

Нельзя is a very common Russian word meaning:

• it is not allowed
• one must not
• you cannot
  depending on context

In this sentence, it expresses prohibition: using the elevator is not allowed during the drill.

Russian often uses нельзя + infinitive:

• нельзя курить — smoking is not allowed
• нельзя входить — do not enter / entering is not allowed
• нельзя пользоваться лифтом — do not use the elevator

A native English speaker may want to translate it with must not, and that is often the best idea here.

Because this is an impersonal construction.

Russian often leaves out the subject when the meaning is general:

• нельзя пользоваться лифтом = one must not use the elevator / you must not use the elevator

It applies to anyone in that situation, so no explicit you is needed.

If you wanted to make it personal, you could say:

• Вам нельзя пользоваться лифтом. — You are not allowed to use the elevator.

But on signs, rules, and announcements, the impersonal version is very common.

Both can be possible, but пользоваться лифтом is very natural and idiomatic.

The verb пользоваться means to use, to make use of. It is commonly used for:

• devices
• services
• transport
• facilities

Examples:

• пользоваться телефоном — to use a phone
• пользоваться словарём — to use a dictionary
• пользоваться лифтом — to use an elevator

Использовать also means to use, but it can sound a bit more neutral, technical, or formal in some contexts.

A very important grammar difference:

• пользоваться takes the instrumental case
• использовать takes the accusative case

So:

• пользоваться лифтом
• использовать лифт

Because пользоваться requires the instrumental case.

The noun is:

• лифт — elevator

Its instrumental singular form is:

• лифтом

So:

• пользоваться лифтом — to use the elevator

This is just the grammar pattern of the verb пользоваться.

More examples:

• пользоваться компьютером — to use a computer
• пользоваться картой — to use a map
• пользоваться ручкой — to use a pen

In this sentence, it mainly means must not or is not allowed to.

Sometimes нельзя can mean literal impossibility, but very often it expresses a rule or prohibition.

Here the meaning is clearly:

• Do not use the elevator during the drill not
• It is physically impossible to use the elevator

So for this sentence, think of нельзя as a rule word.

Yes. Russian word order is flexible.

The sentence as written:

• Во время учебной тревоги нельзя пользоваться лифтом.

This is very natural because it starts with the time condition: during the drill.

You could also say:

• Лифтом нельзя пользоваться во время учебной тревоги.

That version puts more emphasis on лифтом.

Both are grammatical. The original is especially natural for a notice or instruction.

A natural stress pattern would be:

Во врЕмя учЕбной тревОги нельзя́ пОльзоваться лифтОм.

Approximate pronunciation:

• во ВРЕ-мя
• у-ЧЕБ-ной
• тре-ВО-ги
• нель-ЗЯ
• ПОЛЬ-зо-вать-ся
• лиф-ТОМ

The most important thing for a learner is to notice that:

• время has stress on the first syllable: врЕмя
• тревоги has stress on -во-: тревОги
• лифтом has stress on the second syllable: лифтОм