عندما لا أستعمل المصعد، أصعد الدرج إلى الطابق الثالث.

عندما means when. In this sentence, it introduces a time clause:

• عندما لا أستعمل المصعد = when I don’t use the elevator

In context, it often has a habitual sense, so the sentence can also feel like:

• Whenever I don’t use the elevator, I go up the stairs to the third floor.

Compared with إذا, عندما is more clearly a time expression (when), while إذا can often mean if/when depending on context.

لا is the standard way to negate the present/imperfect verb in Modern Standard Arabic.

So:

• أستعمل = I use
• لا أستعمل = I do not use / I don’t use

Here it gives a present or habitual meaning: when I don’t use the elevator.

أستعمل means I use.

It comes from the verb استعمل and is the 1st person singular imperfect form:

• استعمل = to use
• أستعمل = I use

A very common synonym is أستخدم:

• أستخدم المصعد = I use the elevator

Both are fine in MSA.

Because the verb already shows the subject.

In Arabic, the form أستعمل already means I use, and أصعد already means I go up / I climb. So Arabic usually does not need a separate subject pronoun unless you want emphasis.

• أستعمل = I use
• أنا أستعمل = I use / I use

So the sentence does not need أنا.

المصعد means the elevator.

Arabic often uses the definite article الـ where English might say either the elevator or even something more general like the elevator in a building context. Since this sentence is talking about the elevator as a known thing in the situation, the definite form is natural.

• مصعد = an elevator
• المصعد = the elevator

أصعد is the standard MSA verb here and means I go up, I ascend, or I climb.

• أصعد الدرج = I go up the stairs / I climb the stairs

This is a good Modern Standard Arabic choice.
Learners may hear dialect forms like أطلع or بطلع, but those are not the standard MSA form used in this sentence.

So for MSA, أصعد is exactly what you would expect.

Because أصعد can take a direct object in Arabic.

So:

• أصعد الدرج literally means I ascend the stairs
• Natural English translation: I go up the stairs / I climb the stairs

English usually needs a preposition like up, but Arabic does not have to use one here.

الدرج means the stairs or the staircase.

Depending on context, it can refer to:

• the stairs as a set of steps
• the staircase in a building

So أصعد الدرج is a natural way to say I take the stairs / I go up the stairs.

إلى means to, and it marks the destination.

So:

• أصعد الدرج = I go up the stairs
• إلى الطابق الثالث = to the third floor

Together:

• أصعد الدرج إلى الطابق الثالث = I go up the stairs to the third floor

The phrase after إلى tells you where the movement ends.

Because طابق is a masculine noun, so the ordinal adjective must also be masculine.

• الطابق = the floor/story
• الثالث = third (masculine)

Arabic adjectives and ordinals agree with the noun in gender, number, and definiteness.

So:

• الطابق الثالث = the third floor

If the noun were feminine, you would use الثالثة instead.

It most naturally sounds like a habitual or general statement.

Because it uses:

• عندما = when
• present-tense verbs: لا أستعمل, أصعد

the sense is usually:

• When/whenever I don’t use the elevator, I go up the stairs to the third floor.

So it sounds like something the speaker regularly does, not just one single event.

In fully vocalized MSA, it would be:

عندما لا أستعملُ المصعدَ، أصعدُ الدرجَ إلى الطابقِ الثالثِ.

The endings are there because of grammar:

• أستعملُ — present verb
• المصعدَ — direct object
• أصعدُ — present verb
• الدرجَ — direct object
• إلى الطابقِ الثالثِ — genitive after إلى

In normal writing, these short vowel endings are usually omitted.

Yes. That is a very natural English translation.

A few good translations are:

• When I don’t use the elevator, I go up the stairs to the third floor.
• When I don’t use the elevator, I take the stairs to the third floor.
• Whenever I don’t use the elevator, I climb the stairs to the third floor.

All of these match the Arabic well.