Ben merdivenlerden aşağı koşuyorum.

Yes. In Turkish, koşuyorum already includes I because the ending -um marks first person singular.

So:

• Koşuyorum = I am running
• Ben koşuyorum = I am running

Adding ben is usually for:

• emphasis
• contrast
• clarity

For example, Ben koşuyorum, o yürüyüyor means I’m running, he/she is walking.

In this sentence, ben is grammatical but optional.

Merdivenlerden breaks down like this:

• merdiven = stairs / staircase / ladder
• -ler = plural
• -den = from

So literally, merdivenlerden means from the stairs or off the stairs.

In this sentence, it works together with aşağı to give the idea down the stairs.

In Turkish, merdiven can refer to a staircase or stairs, but using the plural merdivenler is very common when talking about stairs as a set of steps.

So:

• merdiven = staircase / stairs
• merdivenler = stairs

Then:

• merdivenlerden aşağı = down the stairs

This is very natural Turkish.

The suffix -den is the ablative case, which often means:

• from
• out of
• off
• sometimes a point of movement away from

Here it helps express movement downward along or from the stairs.

So merdivenlerden aşağı is literally something like from the stairs downward, but the natural English meaning is down the stairs.

Aşağı means down, below, or downward, depending on context.

In this sentence, it functions like a direction word:

• aşağı koşuyorum = I am running down
• merdivenlerden aşağı koşuyorum = I am running down the stairs

It tells you the direction of the movement.

Both can be possible, but they are not used in exactly the same way.

• aşağı = down / downward
• aşağıya = to downward / down to a lower place

With motion verbs, Turkish often uses bare direction words like aşağı, yukarı, içeri, dışarı very naturally:

• aşağı inmek = to go down
• yukarı çıkmak = to go up
• içeri girmek = to go inside

So merdivenlerden aşağı koşuyorum is very natural for I am running down the stairs.

If you said aşağıya, it would sound more like emphasizing the destination to downstairs / to a lower place rather than just the downward motion.

Turkish normally puts the main verb at the end of the sentence.

So the structure here is:

• Ben = subject
• merdivenlerden aşağı = direction/location phrase
• koşuyorum = verb

This is very typical Turkish word order.

A very literal word-for-word order would be:

• I stairs-from down am-running

Even though Turkish word order is somewhat flexible, putting the verb last is the most neutral and common pattern.

Koşuyorum is in the present continuous.

It means:

• I am running
• sometimes, depending on context, I run in a current/immediate sense

In this sentence, it clearly means I am running right now.

It breaks down like this:

• koş- = run
• -uyor- = present continuous suffix
• -um = I

So:

• koşuyorum = I am running

The present continuous suffix is often taught as -(I)yor, and the vowel changes according to vowel harmony:

• geliyorum = I am coming
• bakıyorum = I am looking
• koşuyorum = I am running
• görüyorum = I am seeing

Turkish does not have articles like the and a/an in the same way English does.

So merdivenlerden can mean:

• from stairs
• from the stairs

The exact meaning depends on context.

In this sentence, English naturally uses the stairs, but Turkish does not need a word corresponding to the.

Yes. Turkish often uses this kind of pattern:

• a noun with a case ending
• plus a direction word

For example:

• pencereden dışarı baktım = I looked out the window
• kapıdan içeri girdi = he/she went in through the door
• merdivenlerden aşağı koşuyorum = I am running down the stairs

So this structure is very useful to learn.

Yes, if the context already makes it clear where you are moving.

• Aşağı koşuyorum = I am running down / downstairs
• Merdivenlerden aşağı koşuyorum = I am running down the stairs

The longer version gives more detail.

Koşmak specifically means to run.

So this sentence means that the speaker is moving quickly, by running.

If you only mean I’m going downstairs or I’m coming down the stairs, Turkish would more naturally use inmek:

• Merdivenlerden iniyorum = I am going down the stairs / I am coming down the stairs

So:

• koşuyorum = I am running
• iniyorum = I am going down / descending

The choice depends on whether the idea of running is important.

Yes, Turkish allows some flexibility, but the meaning focus may change.

For example:

• Ben merdivenlerden aşağı koşuyorum. = neutral
• Merdivenlerden aşağı koşuyorum. = also very natural
• Aşağı merdivenlerden koşuyorum. = much less natural in this context

The most natural order here is the original one, with the verb at the end and merdivenlerden aşağı kept together.