По выходным мы часто ездим за город.

По выходным means on weekends or at weekends.

Here выходным is in the dative plural. After по, Russian often uses the dative in expressions of repeated time:

• по утрам — in the mornings
• по вечерам — in the evenings
• по выходным — on weekends

So this structure suggests something that happens regularly.

Because the sentence talks about weekends in general, not one specific weekend.

• по выходным = on weekends / every weekend
• If you meant one particular weekend, you would usually use a different expression, such as в выходные depending on context.

The plural helps show a habitual, repeated action.

This is a very common Russian verbs-of-motion question.

• ехать / едем is for going by transport in one specific direction right now / on one particular trip
• ездить / ездим is for going by transport regularly, habitually, or back and forth

Since the sentence says часто and по выходным, it describes a habit. That is why Russian uses ездим:

• Мы часто ездим за город. — We often go out of town.
• Мы едем за город. — We are going out of town. / We’re heading out of town.
  (one trip, not a general habit)

Because ездить is used when the movement is by vehicle/transport or is thought of that way.

In Russian:

• идти / ходить = go on foot
• ехать / ездить = go by vehicle

So ездим за город usually suggests traveling by car, train, bus, etc., or at least not focusing on walking.

За город literally means beyond the city.

In natural English, it often means:

• out of town
• to the countryside
• outside the city

It does not usually mean going to another city. It suggests leaving the city and going somewhere outside it, often for отдых, nature, a dacha, and so on.

Because the idea is movement to a place outside the city limits.

• в город = into the city
• из города = out of the city / from the city
• за город = outside the city, beyond the city

So ездить за город is a very common Russian expression meaning to go out of town.

After за in the meaning of movement to/beyond, Russian uses the accusative case.

So in за город, город is accusative.

It looks unchanged because город is an inanimate masculine noun, and for many such nouns:

• nominative singular = accusative singular

So:

• nominative: город
• accusative: город

The form is the same, but the function/case is different.

Yes. Russian often drops subject pronouns when they are clear from the verb ending.

• Ездим за город. = We go out of town.
• Мы ездим за город. = We go out of town.

The ending -им already tells you it is we.

Including мы can add:

• emphasis
• contrast
• clarity

So the sentence with мы sounds perfectly normal, but it is not always necessary.

Часто is quite flexible, but here its position is very natural:

• По выходным мы часто ездим за город.

This sounds neutral: On weekends, we often go out of town.

You may also hear other orders, depending on emphasis, for example:

• Мы часто ездим за город по выходным.
• Часто мы по выходным ездим за город.

Russian word order is more flexible than English, but the version in your sentence is very standard and natural.

Yes, but the meaning is a little different.

• по выходным = on weekends, regularly, every weekend
• на выходных = over the weekend / on the weekend, often referring to a particular weekend or speaking more colloquially

So in this sentence, because the action is habitual and repeated, по выходным is the best choice.

The main stresses are:

• По выходны́м
• мы ча́сто
• е́здим or more commonly езди́м?
  Standard stress is е́здим
• за го́род

So the full sentence is:

По выходны́м мы ча́сто е́здим за го́род.

Stress matters in Russian, so it is good to learn expressions like this as whole chunks.