Я медленно спускаюсь по лестнице.

Because спускаться / спуститься is the verb meaning to go down / to descend oneself.

• Я спускаюсь = I am going down
• Я спускаю would come from спускать, which usually means to lower something or to let something down

So:

• Я спускаюсь по лестнице = I am going down the stairs
• Я спускаю чемодан по лестнице = I am lowering / taking the suitcase down the stairs

The ending -юсь here is the 1st person singular present-tense form of the reflexive verb спускаться.

The -ся / -сь ending often makes a verb reflexive, but in many cases it is just part of the dictionary form of a verb and gives it a specific meaning.

Here:

• спускать = to lower something
• спускаться = to go down, descend

So in this sentence, -сь is not really translated as myself in English. It is simply part of the verb спускаться.

The -сь form appears instead of -ся because it comes after a vowel:

• спускаю + сь → спускаюсь

Russian often uses по + dative to mean movement along, over, or via a surface/path.

So:

• по лестнице = along the stairs / by way of the stairs

With verbs like идти, бежать, подниматься, спускаться, this is a very common pattern:

• идти по дороге = to walk along the road
• подниматься по лестнице = to go up the stairs
• спускаться по лестнице = to go down the stairs

In English we usually say down the stairs, but Russian expresses it with по.

Because по here requires the dative case.

The noun is:

• dictionary form: лестница = staircase, stairs

Its dative singular form is:

• лестнице

So:

• по лестнице = along/down the stairs

This is a very standard case pattern, not something special to memorize only for this sentence.

That is a natural, neutral word order in Russian.

• Я медленно спускаюсь по лестнице sounds like a straightforward statement.
• Russian word order is flexible, so other versions are possible:
  • Я спускаюсь по лестнице медленно
  • Медленно я спускаюсь по лестнице

But the emphasis changes a little.

Usually:

• медленно before the verb = neutral, natural
• медленно later in the sentence = can sound more emphatic or contrastive depending on context

So the original order is a good default.

Yes, it can be omitted.

Because the verb ending already shows the subject:

• спускаюсь = I am going down

So Russian often drops the pronoun when it is clear:

• Медленно спускаюсь по лестнице.

Including Я is still completely correct. It may sound:

• slightly more explicit
• more natural at the start of a sentence
• useful for contrast, for example: Я медленно спускаюсь, а он бежит.

So both are possible.

Grammatically, it is present tense imperfective.

That can mean:

1. right now / in progress

   • Я медленно спускаюсь по лестнице.
   • I am going down the stairs slowly.

2. a repeated or usual action, depending on context

   • for example, in a story or routine description

In this sentence by itself, most learners will understand it as an action happening now.

Спускаюсь is imperfective.

Russian uses the imperfective here because the action is viewed as:

• ongoing
• in progress
• not presented as completed

That matches English I am going down very well.

The perfective partner is спуститься:

• Я спущусь по лестнице = I will go down the stairs / I’ll go down the stairs

So:

• спускаюсь = process
• спущусь = completed single action in the future

You usually would not say иду по лестнице in this exact sense without more context, because спускаться already gives the important direction: downward.

• спускаться = to descend, go down
• подниматься = to ascend, go up

You can say something like:

• Я медленно иду вниз по лестнице.

That also means I am slowly going down the stairs, but it is built differently:

• иду = I am going
• вниз = downward
• по лестнице = along/down the stairs

The version with спускаюсь is often more compact and natural.

It can mean either, depending on context.

In English, we often use stairs where Russian uses singular лестница.

So:

• по лестнице can be translated as down the stairs
• or down the staircase

In everyday translation, stairs is usually the most natural English choice.

Yes, you could, but вниз is often unnecessary because спускаться already means to go down.

So:

• Я медленно спускаюсь по лестнице = natural and sufficient
• Я медленно спускаюсь вниз по лестнице = possible, but more emphatic or a bit redundant

Russian sometimes allows this kind of doubling for emphasis, but the simpler version is usually better for learners.