Si le métro est en panne, je prends le tram pour aller au centre-ville.

Yes. Si here means if and introduces a condition:

Si le métro est en panne, je prends le tram...
= If the subway is out of order, I take the tram...

This is a very common way to make if sentences in French.

A useful pattern is:

Si + present, present

In this sentence:

• si le métro est en panne = if the metro is broken / not running
• je prends le tram = I take the tram

This structure often expresses a habitual or general reaction: whenever that situation happens, this is what I do.

French often uses the present tense in both parts when talking about:

• a general rule
• a habit
• something that usually happens

So:

Si le métro est en panne, je prends le tram.

means something like:

• If the metro is down, I take the tram
• When the metro is down, I take the tram
• If the metro is down, I’ll take the tram in a habitual sense

If you wanted a more specific future meaning, French could also say:

Si le métro est en panne, je prendrai le tram.
= If the metro is down, I’ll take the tram.

So the original sentence sounds like a usual practice, not just one future event.

Être en panne means to be broken down, to be out of order, or to not be working.

So:

• Le métro est en panne = the metro is not working / the metro system is down

Very important: en panne is a fixed expression. French commonly uses it for machines, vehicles, elevators, appliances, transport systems, etc.

Examples:

• Ma voiture est en panne. = My car has broken down.
• L’ascenseur est en panne. = The elevator is out of order.
• Internet est en panne. = The internet is down.

Both are possible, but they mean slightly different things.

• Je prends le tram = I take the tram
• Je vais en tram = I go by tram

In your sentence, French uses prendre because it focuses on the transport choice, just like English take:

• prendre le métro
• prendre le bus
• prendre le train
• prendre le tram

So je prends le tram is very natural here.

In French, means of transport often use the definite article:

• le métro
• le tram
• le bus
• le train

And after prendre, French usually says:

• prendre le métro
• prendre le bus
• prendre le tram

English often says take the subway / the bus, so this is similar.

French generally does not say je prends tram. The article is needed here.

Métro is the noun itself: metro / subway.

But in a full sentence, French usually needs an article or another determiner:

• le métro = the metro
• un métro = a metro train / a metro
• ce métro = this metro

So in your sentence, le métro refers to the metro system as the mode of transport.

The same is true for le tram.

Pour means to or in order to here, and aller means to go.

So:

je prends le tram pour aller au centre-ville
literally = I take the tram in order to go to the city center

In natural English, we often just say:

• I take the tram to go downtown
• or simply I take the tram downtown

French often uses pour + infinitive to show purpose:

• Je téléphone pour demander un renseignement. = I’m calling to ask for information.
• Il vient pour travailler. = He’s coming to work.

Because à + le contracts to au in French.

So:

• à + le = au
• à + les = aux

That means:

• aller au centre-ville = to go to the city center
• not aller à le centre-ville

Other examples:

• Je vais au bureau. = I’m going to the office.
• Il est au restaurant. = He’s at the restaurant.

Centre-ville means city center, town center, or sometimes downtown, depending on context.

It refers to the central part of a town or city.

Examples:

• J’habite en centre-ville. = I live downtown / in the city center.
• Il y a beaucoup de magasins au centre-ville. = There are lots of shops in the city center.

It is often written with a hyphen: centre-ville.

Yes. French allows that.

You can say:

• Si le métro est en panne, je prends le tram pour aller au centre-ville.
• Je prends le tram pour aller au centre-ville si le métro est en panne.

Both are correct.

When the si clause comes first, a comma is commonly used:

• Si le métro est en panne, ...

When it comes second, the comma is often not needed:

• Je prends le tram ... si le métro est en panne.

By itself, it most naturally sounds like a habit or usual reaction:

If the metro is down, I take the tram to go downtown.

It suggests:

• this is what I normally do
• this is my standard solution

If you want to make it clearly about one future situation, French often prefers:

• Si le métro est en panne, je prendrai le tram.

If you want to make it clearly about right now, context would help:

• Le métro est en panne, alors je prends le tram. = The metro is down, so I’m taking the tram.

Because French normally needs the preposition à with aller when you say where someone is going:

• aller à Paris
• aller au centre-ville
• aller à l’école
• aller au bureau

So aller centre-ville is not standard French.

The full structure is:

• aller à + place
• which becomes aller au before a masculine singular noun with le

Yes, it’s a little tricky.

Je prends is pronounced roughly like:

zhuh prahn

A few points:

• the s in prends is silent
• the d is also silent
• the en gives a nasal sound, not a full en like in English

So although it is spelled prends, you do not pronounce all the final letters.

Also:

• je prends
• tu prends
• il prend

sound very similar, because the final consonants are usually silent.

Yes. Tram and tramway both exist.

• le tram is the common everyday form
• le tramway is more formal or full-length

In normal speech, people very often say tram:

• Je prends le tram.

So the sentence sounds completely natural as it is.

Not exactly.

• si = if
• quand = when

If you say:

• Si le métro est en panne...
  you mean if the metro is down

If you say:

• Quand le métro est en panne...
  you mean when the metro is down

Sometimes the meanings are close, especially in habitual situations, but they are not identical:

• si expresses a condition
• quand suggests the situation does happen

So si is the right word if the idea is conditional: if that happens, then I do this.