Jedna paní na zastávce mi vysvětlila, kterým směrem jede metro do centra.

Yes, jedna is literally one, but in sentences like this it often works more like a, one, or a certain in English.

So Jedna paní can mean:

• a lady
• one lady
• this one lady / a certain lady

Czech has no articles like a or the, so words like jeden / jedna / jedno are often used when the speaker is introducing someone new into the story.

Paní is a polite word for lady, woman, or sometimes Mrs.

In this sentence, jedna paní most naturally means a lady or a woman.

It does not necessarily mean she was old; it is just a polite, normal way to refer to an adult woman.

Because na meaning at/on for location takes the locative case.

• base form: zastávka = stop
• locative singular: na zastávce = at the stop

So:

• na zastávce = at the stop
• na stanici = at the station

Here zastávka usually suggests a bus or tram stop rather than a metro station.

Mi means to me and it is the short dative form of já.

• mně = full/stronger form
• mi = short unstressed form

So:

• mi vysvětlila = explained to me

Czech often uses the dative for the person receiving an explanation, message, gift, help, etc.

Because mi is a clitic: a short unstressed word that usually goes in the second position in the clause.

In Czech, second position means after the first unit of meaning, not necessarily after the first single word.

Here the first unit is:

• Jedna paní na zastávce

So the clitic comes next:

• Jedna paní na zastávce mi vysvětlila...

This word order is very natural in Czech.

Because the subject paní is feminine singular, and Czech past tense agrees with the subject in gender and number.

Compare:

• vysvětlil = he explained
• vysvětlila = she explained
• vysvětlilo = it explained
• vysvětlili / vysvětlily = they explained

Since paní is feminine, the verb is vysvětlila.

Because Czech often leaves out subject pronouns when they are clear from the verb form or the context.

Here paní is already the subject, so there is no need to add ona (she).

Also, in the past tense, the form vysvětlila itself already shows feminine singular, which supports the meaning she explained.

So Czech naturally says:

• Paní mi vysvětlila...

not usually:

• Paní ona mi vysvětlila...

Because kterým směrem jede metro do centra is a subordinate clause.

It is an indirect question, introduced by kterým směrem = which direction / which way.

Czech normally separates subordinate clauses with a comma, so:

• ... mi vysvětlila, kterým směrem jede metro do centra.

That comma is required.

Both words are in the instrumental singular:

• který → kterým
• směr → směrem

The phrase kterým směrem is a very common expression meaning:

• which direction
• which way
• in which direction

So this is best learned as a set phrase:

• Kterým směrem...? = Which way...?

Because the explanation happened in the past, but the direction the metro goes is presented as a current or general fact.

So the timeline is:

• in the past: the lady explained it
• in the present/general sense: the metro goes that way

This is completely normal in both Czech and English:

• She explained which way the metro goes to the center.

The verb inside the subordinate clause stays in the present because it describes the route, not the moment of explaining.

Because jet is the normal Czech verb for going/running by vehicle.

You use jet for things like:

• trains
• buses
• trams
• cars
• metro

So:

• Metro jede do centra. = The metro goes/is going to the center.

It would not be natural to use jít here, because jít is for going on foot.

The preposition do takes the genitive case, so centrum changes to centra.

• nominative: centrum
• genitive: centra

So:

• do centra = to the center / into the center

This is a very common movement pattern in Czech:

• do školy = to school
• do města = to the city
• do centra = to the center

The sentence could be rearranged somewhat, but the original order is very natural.

Original:

• Jedna paní na zastávce mi vysvětlila, kterým směrem jede metro do centra.

Why it works well:

• Jedna paní na zastávce introduces the person and place
• mi comes early because it is a clitic
• vysvětlila gives the main action
• the subordinate clause comes after it

You might also hear variations for emphasis, but not all of them sound equally neutral. The given version is a good standard model.