Merdivenden inerken dizimi vurdum, ama ciddi değil.

-den / -dan is the ablative ending, which often means from, off, or out of.

So:

• merdiven = stairs / staircase
• merdivenden = from the stairs / down the stairs

In this sentence, Merdivenden inerken means something like while going down the stairs.

A useful point: Turkish often uses merdiven in a collective sense, so even though English usually says stairs (plural), Turkish may use merdiven as a single word for the whole staircase.

İnerken comes from the verb inmek = to go down / descend.

Breakdown:

• in- = verb stem
• -erken = while doing, when doing

So:

• inerken = while going down, when going down

This form is used when one action is happening at the same time as another:

• Merdivenden inerken dizimi vurdum = I hit my knee while going down the stairs

It gives background action for the main event.

It actually is inerken: inmek loses the -mek infinitive ending, and the stem behaves as in-. Before -erken, Turkish uses the present/aorist-style form here:

• inmek → inerken

This same pattern appears in many verbs:

• gelmek → gelirken = while coming
• gitmek → giderken = while going
• okumak → okurken = while reading

So inerken is the normal and correct form.

Dizimi breaks down like this:

• diz = knee
• -im = my
• -i = definite direct object marker

So:

• dizim = my knee
• dizimi = my knee (as the object of the verb)

Because vurmak here is acting on a specific thing — my knee — Turkish marks it as a definite object.

So dizimi vurdum literally means I hit my knee.

Turkish very often uses a possessive ending with body parts:

• başım = my head
• kolum = my arm
• ayağım = my foot
• dizim = my knee

So when talking about an injury, Turkish naturally says things like:

• Başımı vurdum = I hit my head
• Kolumu incittim = I hurt my arm
• Dizimi vurdum = I hit my knee

This is extremely normal Turkish.

Yes. Vurmak basically means to hit, to strike, or to bang.

In everyday Turkish, it is very natural for accidental impact:

• Başımı kapıya vurdum = I hit my head on the door
• Dirseğimi masaya vurdum = I banged my elbow on the table
• Dizimi vurdum = I hit/banged my knee

So in this sentence, vurdum does not sound violent; it can simply describe an accidental knock or bump.

Turkish often leaves out subject pronouns when they are already clear from the verb ending.

Here:

• vurdum = I hit
• the -um ending already tells you the subject is I

So ben is unnecessary unless you want emphasis:

• Ben dizimi vurdum = I hit my knee

Without ben, the sentence sounds more natural in normal conversation.

Because the meaning is It is not serious, not I am not serious.

• değilim would mean I am not
• değil here means is not

The subject is understood as the injury, situation, or problem:

• (Bu) ciddi değil = This is not serious

Turkish often leaves out pronouns like bu when the context is obvious.

So:

• ama ciddi değil = but it is not serious

Yes. In Turkish, adjective sentences in the present tense often do not need an overt verb equivalent to English is.

So:

• Hasta = He/she is sick
• Mutlu = He/she is happy
• Ciddi değil = It is not serious

In the negative, Turkish uses değil.

So this is a very normal Turkish structure.

The order is very natural Turkish:

• Merdivenden inerken = background/time-setting part
• dizimi = object
• vurdum = main verb
• ama ciddi değil = follow-up comment

Turkish often puts:

1. time/background information first,
2. the main action later,
3. the finite verb near the end of the clause.

So this sentence sounds very natural as written.

Some rearrangement is possible, but it changes emphasis. For example:

• Dizimi merdivenden inerken vurdum
  Still understandable, but the original order is smoother and more typical.

Yes, merdivenlerden inerken is possible and would also mean while going down the stairs.

But merdivenden inerken is very common and natural because merdiven can refer to the staircase as a whole.

So:

• merdivenden inerken = very natural
• merdivenlerden inerken = also possible, with a slightly more literal from the stairs / down the steps feel

Both are understandable; the version in your sentence is completely normal.

Ama means but.

It links the two ideas:

• I hit my knee while going down the stairs
• but it’s not serious

So ama introduces contrast: something bad happened, but the speaker is reassuring the listener that it is not a serious problem.