Learn Hardcore Russian: Сегодня мы едем в центр на трамвае, а по выходным часто ездим туда на метро. - Today we are going to the center by tram, and on weekends we often go there by metro.

ехать → едем = one-time, right now / today, movement in one direction.
- Сегодня мы едем… = Today we are going (this one trip)…
ездить → ездим = habitual / repeated action, from time to time, usually.
- …по выходным часто ездим… = on weekends we (often) go (regularly)…

So the sentence contrasts:

едем for today’s specific trip, and
ездим for the general, repeated weekend habit.

Why is it в центр and not в центре? What’s the difference?

The preposition в can take either:

Accusative (direction: to, into, toward), or
Prepositional (location: in, at).

Here we have:

в центр = to the center / downtown (direction → Accusative: центр → в центр)
в центре = in the center / downtown (location → Prepositional: центр → в центре)

In this sentence we talk about going to the center, so в центр is required.

Why is it на трамвае and на метро? How does на + transport work?

With means of transport, Russian normally uses на + [Instrumental case]:

на трамвае (Instrumental of трамвай)
на автобусе, на машине, на поезде, etc.

Метро is indeclinable (its form doesn’t change), so на метро looks like Nominative but is understood as Instrumental.

Semantically, на + Instrumental here means by / on (a means of transport):

ехать на трамвае = to go by tram
ехать на метро = to go by metro / subway

What exactly does по выходным mean, and why is по used with Dative?

По выходным literally is along / over weekends, and idiomatically means on weekends / at weekends (regularly).

Grammar:

выходной день → plural выходные (дни) = weekends / days off
по выходным uses Dative plural: выходные → выходным

Typical pattern:

по понедельникам – on Mondays
по вечерам – in the evenings
по праздникам – on holidays

So по + Dative plural expresses regular repetition on those days.

Could we say на выходных instead of по выходным? Is there a difference?

Both exist, but they’re not identical:

по выходным — focuses on regularity/habit: on weekends (generally, as a rule).
на выходных — usually about a specific weekend or weekend time frame: this/that weekend, on the weekend.

In this sentence, we’re describing a habit:

On weekends we often go there by metro → по выходным часто ездим… is the natural choice.

Why is there туда after ездим? Isn’t в центр already mentioned?

Туда means there (direction) and refers back to в центр.

First clause: в центр specifies where we are going today.
Second clause: we don’t repeat в центр, so we replace it with туда = to that place (there).

So:

Сегодня мы едем в центр…
…а по выходным часто ездим туда… = …but on weekends we often go there… (to the same place just mentioned)

You could repeat в центр, but туда avoids repetition and sounds more natural.

Why is the conjunction а used instead of и between the two parts?

Both а and и can be translated as and, but:

и = simple addition, and also
а = contrast, comparison, whereas / but / on the other hand

In the sentence, there is a contrast:

Today (one-time action) we go by tram.
On weekends (habitual) we go by metro.

So а correctly marks this contrast:

…на трамвае, а по выходным… на метро.
= …by tram, whereas on weekends… by metro.

What is the difference between ехать / ездить and идти / ходить? Why not use идти here?

Russian has two sets of motion verbs:

On foot:
- идти (one direction, one trip now)
- ходить (habitual / back-and-forth / multi-directional)
By vehicle/transport:
- ехать (one direction, one trip now)
- ездить (habitual / multi-directional)

In this sentence, we are clearly using transport (tram, metro), so we must choose from ехать / ездить, not идти / ходить.

Сегодня мы едем… (by tram, one trip)
…часто ездим… (by metro, regularly)

Why is центр in the Accusative, трамвае in the Prepositional, and выходным in the Dative?

Each preposition requires a specific case:

в центр
- в
  - Accusative = direction / destination
- noun: центр → в центр (Accusative singular)
на трамвае
- with means of transport: на
  - Instrumental
- noun: трамвай → трамваем (Instrumental singular)
- after на, Instrumental is realized as на трамвае (fixed phrase form)
по выходным
- по
  - Dative (for repeated times/days)
- noun: выходные → выходным (Dative plural)

Different roles → different prepositions → different cases.

Could the word order change, for example Мы сегодня едем… or Часто по выходным ездим…?

Yes, Russian word order is flexible, and these variations are possible:

Мы сегодня едем в центр на трамвае… (neutral, slightly more emphasis on мы)
Сегодня мы едем… (neutral, typical narrative start: Today we…)

Second clause:

По выходным мы часто ездим туда на метро.
Мы по выходным часто ездим туда на метро.
Часто по выходным мы ездим туда на метро. (emphasizes often)

The original order is natural and neutral; other orders add slight emphasis but remain correct.

Where does часто usually go in a sentence like this? Could it move?

Часто (often) is an adverb of frequency and is fairly mobile, but it normally appears near the verb or time expression.

Possible positions:

По выходным часто ездим туда на метро. (neutral; often linked to weekends)
Часто по выходным ездим туда на метро. (often slightly emphasized)
По выходным ездим туда на метро часто. (less natural; sounds “tacked on”)

The original по выходным часто ездим is a very natural placement.