Когда я спускаюсь по лестнице, я держусь за перила.

Because спускаться / спуститься means to go down / descend, and in Russian this verb is normally used with -ся when a person is descending.

• я спускаюсь = I am going down / I go down
• я спускаю usually means I lower something or I let something down

So:

• Я спускаюсь по лестнице = I go down the stairs.
• Я спускаю чемодан по лестнице = I lower/carry a suitcase down the stairs.

The -ся form shows that the subject is the one moving downward.

Russian uses по + dative to mean movement along, on, or down/up a surface or route.

So:

• идти по улице = walk along the street
• подниматься по лестнице = go up the stairs
• спускаться по лестнице = go down the stairs

Here, по лестнице means along/down the staircase.

By contrast:

• на лестнице = on the staircase, located there
• в лестнице would usually not make sense here

So по is the normal preposition for this kind of movement.

Because the preposition по here requires the dative case.

The noun changes like this:

• лестница = nominative
• по лестнице = dative

This is a very common pattern:

• по улице
• по дороге
• по мосту
• по лестнице

So the ending -е is there because of the case required by по.

They are related, but not identical.

• спускаться по лестнице = to go down the stairs, focusing on movement along the staircase
• спускаться с лестницы = to come down from the staircase, focusing more on the starting point or source

In your sentence, по лестнице is the natural choice because the action is descending via the stairs.

A simple way to remember it:

• по лестнице = along/down the stairs
• с лестницы = from the stairs

Because держаться за is a set expression meaning to hold on to something.

Compare:

• Я держу сумку. = I am holding a bag.
• Я держусь за перила. = I am holding on to the railing.

So:

• держать
  • direct object = hold something
• держаться за
  • object = hold on to something

The reflexive form держаться changes the meaning.

Because the verb держаться normally uses the pattern держаться за + accusative, meaning to hold on to something.

Examples:

• держаться за руку = hold someone’s hand
• держаться за стену = hold on to the wall
• держаться за перила = hold on to the railing

So here за does not mean literal behind. In this expression, it is part of the construction держаться за, meaning grasp / hold on to.

Перила is one of those Russian nouns that is normally used only in the plural. It refers to a railing / banister / handrail, even when English might use a singular word.

So:

• перила = railing / banister / railings

This is similar to how some English nouns are often plural in form, like scissors or pants.

So за перила does not necessarily mean several separate railings. It can refer to the staircase railing as a whole.

Because the sentence describes a habitual or general action:

• Когда я спускаюсь по лестнице, я держусь за перила.

This means something like Whenever I go down the stairs, I hold on to the railing.

Russian often uses the present tense in both parts of a sentence to talk about repeated actions or usual behavior.

So this is not only about right now. It can mean what someone generally does.

In this sentence, it can naturally be understood as when in a general sense, which in English is often best expressed as whenever.

So the idea is:

• Когда я спускаюсь по лестнице... = When / Whenever I go down the stairs...

Because both verbs are in the present tense and the sentence describes a repeated habit, whenever is often the best way to understand it.

Not necessarily. Russian often omits subject pronouns when the verb already makes the subject clear.

So both are possible:

• Когда я спускаюсь по лестнице, держусь за перила.
• Когда я спускаюсь по лестнице, я держусь за перила.

Including the second я makes the sentence a little clearer or more balanced, but it is not required.

Yes. Russian word order is flexible, though some versions sound more neutral than others.

The original sentence:

• Когда я спускаюсь по лестнице, я держусь за перила.

is very natural and neutral.

You could also say:

• Я держусь за перила, когда спускаюсь по лестнице.

This means the same thing, but the focus shifts slightly to I hold on to the railing.

So the word order can change, but the original version is a standard, natural way to say it.

Спускаюсь is imperfective. It is used here because the sentence describes a repeated, ongoing, or habitual action.

• спускаться = imperfective
• спуститься = perfective

Compare:

• Когда я спускаюсь по лестнице, я держусь за перила.
  = Whenever I go down the stairs, I hold on to the railing.

• Когда я спущусь по лестнице, ...
  = When I have gone down the stairs, ...

The perfective form спущусь would refer to one completed future action, which is not what this sentence is about.