Learn Hardcore Czech: Při cestě do třetího patra někdy čekám na výtah, ale když je večer tma, raději jdu po schodech. - On the way to the third floor I sometimes wait for the elevator, but when it is dark in the evening, I prefer to go by the stairs.

do + genitive = movement into an enclosed space or area
- do třetího patra = into the third floor / up to the third floor (as a level of a building)
na + accusative = movement onto a surface or place you go to (events, islands, etc.)
- na balkon (onto the balcony)
- na koncert (to a concert)

For floors of a building, do (prvního/druhého/třetího) patra is the normal, idiomatic choice.
Na třetí patro would sound odd or wrong in standard Czech.

Why is it třetího patra and not just třetí patro?

Because of the preposition do:

do requires the genitive case.
patro (floor) is neuter:
- nominative: patro
- genitive: patra
třetí is an ordinal adjective (“third”). In the neuter genitive singular, it becomes třetího.

So:

nominative: třetí patro (the third floor)
genitive (after do): do třetího patra (to the third floor)

Why is někdy placed before čekám? Could we also say čekám někdy na výtah?

Někdy means “sometimes” and is fairly flexible in position.

The most natural positions are:

Někdy čekám na výtah.
Čekám někdy na výtah.

Both are grammatically correct. The default, neutral word order is often:

Někdy čekám na výtah.

Putting někdy at the start makes it feel like you’re emphasising the time-frequency (“Sometimes I wait…”).
Čekám někdy na výtah is also okay, perhaps a bit more “inside the sentence,” but not wrong.

Why do we say čekám na výtah and not just čekám výtah?

In Czech, čekat almost always takes preposition + object, not a bare accusative like in English.

čekat na něco/někoho = to wait for something/someone
- čekám na výtah = I wait for the elevator
- čekám na autobus = I wait for the bus
- čekám na tebe = I wait for you

Čekám výtah is ungrammatical in standard Czech. You need na + accusative: na výtah.

Why is the subject já (“I”) not stated in čekám and jdu?

Czech is a pro-drop language: subject pronouns (já, ty, on, ona, etc.) are often omitted because the verb ending already shows the person:

čekám = I wait / I am waiting
jdu = I go / I am going

You can say já čekám or já jdu if you want to emphasise I (e.g. contrast: I do it, not someone else), but in a neutral sentence it’s more natural to leave já out:

Někdy čekám na výtah…
…raději jdu po schodech.

In ale když je večer tma, what is večer grammatically, and why that form?

Večer here is a time expression meaning “in the evening”.

Grammatically:

It is the noun večer (evening) in the accusative, used as an adverbial of time.
Czech often uses bare accusative nouns to express “when”:
- Včera (yesterday), dnes (today), zítra (tomorrow) are similar.
- Večer čtu. = I read in the evening.

So když je večer tma = when it is dark in the evening.
You could also say když je tma večer, but když je večer tma is more common and natural.

Why is it je tma (with the noun tma) instead of an adjective like je tmavý or je tmavo?

Czech commonly uses the noun tma (darkness) in this pattern:

Je tma. = It is dark. (literally “There is darkness.”)

Compare:

Je světlo. = It is light / it’s bright.
Je zima. = It is cold / it’s winter / it’s cold weather.

Je tmavý would describe a specific object:

Ten pokoj je tmavý. = That room is dark.

Je tmavo is also possible:

Večer je tam dost tmavo. = It’s quite dark there in the evening.
Here tmavo is an adverb-like form.

In your sentence, je večer tma is the very standard, neutral way to say “it’s dark in the evening.”

What exactly does raději mean, and how is it different from radši or spíš?

Raději is the comparative form of rád (“gladly”), and means “rather, preferably, I’d prefer to…”.

Raději jdu po schodech.
= I prefer to take the stairs. / I’d rather go by the stairs.

Variants:

raději – neutral / slightly more formal
radši – very common in speech, a bit more colloquial
- Radši jdu po schodech.
spíš / spíše – “rather / more likely”; close in meaning, but a bit broader:
- Spíš jdu po schodech. = I’m more likely to go by the stairs / I tend to take the stairs.

In this sentence, raději and radši are almost interchangeable in everyday language.

Could we also say Jdu raději po schodech instead of raději jdu po schodech? Does the word order change the meaning?

Yes, both are correct:

Raději jdu po schodech.
Jdu raději po schodech.

The meaning is basically the same: I’d rather take the stairs.

Subtle nuance:

Raději jdu po schodech – slight emphasis on the preference (“Rather, I go by the stairs”).
Jdu raději po schodech – feels a bit more like you are stating how you go (you go, and the way you go is “rather by the stairs”).

In everyday speech, they are both natural and understood the same way.

Why is it po schodech and not something like po schody or po schodách?

The preposition po in the sense of “along / over / on (a surface)” takes the locative case:

po + locative plural of schod = po schodech

Forms of schod (step/stair):

nominative plural: schody (the stairs)
locative plural: schodech

So:

jdu po schodech = I walk on/along the stairs / up the stairs

Po schody would be accusative and is incorrect in this meaning.
Po schodách is a common dialectal/colloquial form; in standard Czech, po schodech is preferred.

Why do we use čekám (imperfective) instead of počkám (perfective) here?

Čekám (imperfective) describes an ongoing or habitual action:

někdy čekám na výtah = I sometimes (tend to) wait for the elevator
→ a regular behaviour, not a single completed act.

Počkám (perfective) focuses on completion of one act of waiting:

Počkám na výtah. = I’ll wait for the elevator (this time, until it comes).

In your sentence, you’re talking about what you sometimes do in general, so the imperfective čekám is correct and natural.

Why is it jdu and not chodím in raději jdu po schodech?

Both verbs mean “to go (on foot)”, but there is a nuance:

jít / jdu = to go once, in a single direction (one specific trip).
chodit / chodím = to go repeatedly, habitually, or back and forth.

Your sentence as written can be understood as describing a typical choice in a given situation (on that kind of trip up to the third floor, this is what you choose), so jdu is fine.

If you want to stress that this is your general habit whenever you go to the third floor, you could also say:

…raději chodím po schodech.
= I generally prefer to use the stairs / I usually take the stairs.

Both are possible; jdu sounds a bit more like one concrete decision in that situation, chodím more like a standing habit.