Learn Hardcore Russian: Когда лифт не работает, я держусь за перила и медленно поднимаюсь по лестнице. - When the elevator doesn’t work, I hold on to the handrail and slowly climb the stairs.

Когда here means whenever / when(ever) and sets a regular situation: When the elevator doesn’t work (in general), I do X.
Если means if and sounds more conditional/hypothetical: If the elevator doesn’t work (this time), I’ll do X.
Both can be grammatical, but they imply slightly different attitudes: Когда = repeated/typical; Если = conditional.

Why is there a comma after Когда лифт не работает?

Because Когда лифт не работает is a subordinate clause placed before the main clause. In Russian, a leading subordinate clause is normally separated by a comma:
Когда …, я …

Why is не placed before работает (and not elsewhere)?

Russian не typically goes directly before the word it negates. Here it negates the verb работает:
лифт не работает = the elevator is not working.

Why is работает in the present tense—doesn’t it refer to “when it doesn’t work” in general?

Russian present tense often expresses habitual/general time, like English present simple:
Когда лифт не работает = When/Whenever the elevator doesn’t work (general recurring situation).
It can also describe a current situation if the context makes it “right now.”

Is я necessary here, or can it be dropped?

It can be dropped:
Когда лифт не работает, держусь за перила и медленно поднимаюсь по лестнице.
Russian verb endings already show the subject (1st person singular). Я is kept for clarity, emphasis, or contrast.

What does держусь mean exactly, and why is it reflexive?

держусь is from держаться (reflexive) = to hold on (to something), to keep one’s grip.
Compare:

держать (non‑reflexive) = to hold something (I hold it)
держаться (reflexive) = to hold on / to cling (I hold on)

So я держусь means I’m holding on (for support).

Why is it держусь за перила—what case is перила, and why за?

With держаться, the common pattern is держаться за + Accusative (hold on to something).
перила is in the accusative here, but it looks the same as nominative because перила is a plural-only noun (no singular form in normal use).

What are перила grammatically—are they singular or plural?

перила is plural-only (pluralia tantum): it behaves as plural grammatically.
So you say:

эти перила (these railings)
перила (railings/handrail as a set)

Why is поднимаюсь also reflexive?

подниматься (imperfective) / подняться (perfective) are commonly used as reflexive verbs meaning to go up / to ascend (yourself).
Non-reflexive поднимать means to lift/raise something (an object), e.g. поднимать сумку = to lift a bag.

Why is it медленно поднимаюсь—can the adverb go elsewhere?

Yes, adverbs are flexible. Common options:

медленно поднимаюсь (neutral)
поднимаюсь медленно (also neutral, slightly more emphasis on slowly)
я медленно поднимаюсь (emphasis on I doing it slowly)

Why is it по лестнице and not something like на лестницу?

по + Dative often means along / via / by means of a path:
поднимаюсь по лестнице = I go up via the stairs / along the staircase.
на лестницу (accusative with на) would mean onto the stairs (movement to the surface/area), focusing on arriving on the stairs rather than moving upward along them.

What case is лестнице in по лестнице?

It’s dative singular. The preposition по (in this “along/by” meaning) requires dative:
по лестнице (dative of лестница).

Why are держусь and поднимаюсь both in the imperfective present?

Imperfective present fits a repeated/ongoing process: I (generally) hold on and (slowly) go up.
Perfective would usually point to a completed single action, more like I will go up (and finish), e.g. поднимусь in a future/one-time context.