Почему бы нам не зайти в кафе после семинара?

Почему бы нам не... is a very common Russian pattern used to make a suggestion, not to ask for a literal reason.

So Почему бы нам не зайти в кафе после семинара? means something like:

• Why don't we stop by a café after the seminar?
• How about going to a café after the seminar?
• Why not go to a café after the seminar?

Even though почему usually means why, in this pattern the whole expression functions like a polite, conversational suggestion.

This is part of the fixed suggestion pattern:

• Почему бы + dative + не + infinitive?

So here:

• Почему бы = why would it not / why not
• нам = to us / for us
• не зайти = not stop by

But you should not translate it word-for-word too literally. The full pattern simply means:

• Why don't we...?
• Why not...?

The не is required in this construction. Without it, the sentence would not sound like the normal Russian suggestion pattern.

Нам is the dative case of мы (we).

This construction uses the dative:

• Почему бы мне не... = Why don't I...?
• Почему бы тебе не... = Why don't you...?
• Почему бы нам не... = Why don't we...?

So нам does not mean the subject we in the usual nominative sense; it is part of the set pattern used for suggestions.

Зайти is a perfective verb. Its basic meaning is something like:

• to drop in
• to stop by
• to go in for a short time

In this sentence, зайти в кафе means:

• to stop by a café
• to pop into a café

It suggests a single, completed visit, which is why the perfective verb fits well here.

These verbs are different in meaning:

• идти в кафе = to go to a café (right now / in one direction)
• ходить в кафе = to go to cafés habitually / in general
• зайти в кафе = to stop by a café, usually briefly or as part of a plan

Here the speaker is suggesting a specific action after the seminar: dropping into a café. That is why зайти sounds natural.

The verb зайти often combines with в + accusative when it means to go into / stop by a place.

So:

• зайти в кафе = stop by a café
• зайти в магазин = stop by a store
• зайти в офис = stop by the office

Here кафе looks the same in different cases because it is an indeclinable noun, so you do not see a form change.

Кафе is an indeclinable noun in Russian. That means its form stays the same in different cases.

For example:

• Я люблю кафе.
• Мы сидим в кафе.
• Пойдём в кафе.

The role of the word is shown by the preposition and context, not by a changed ending.

The preposition после requires the genitive case.

So:

• семинар → nominative
• после семинара → genitive

Other examples:

• после урока = after the lesson
• после работы = after work
• после встречи = after the meeting

So после семинара simply means after the seminar.

It tells you when the suggested action should happen.

The sentence is not just Why don’t we stop by a café?
It is specifically:

• Why don’t we stop by a café after the seminar?

In Russian, time expressions like this often come at the end, though word order can change for emphasis.

Yes. Russian word order is more flexible than English word order.

The neutral version is:

• Почему бы нам не зайти в кафе после семинара?

But you could also hear variations such as:

• Почему бы нам после семинара не зайти в кафе?
• Почему бы нам не зайти после семинара в кафе?

These all mean roughly the same thing, but the emphasis shifts a little:

• после семинара earlier in the sentence can highlight the time
• в кафе later can make the destination more prominent

The original sentence sounds natural and neutral.

It sounds natural, polite, and conversational. It is a friendly way to make a suggestion.

It is softer than a direct imperative like:

• Пойдём в кафе после семинара. = Let’s go to a café after the seminar.

And softer than a blunt question like:

• Мы пойдём в кафе после семинара? = Are we going to a café after the seminar?

So Почему бы нам не... is a good pattern when you want to suggest something without sounding pushy.

Both can suggest an action, but the tone is different.

• Почему бы нам не зайти в кафе после семинара?
  = Why don’t we stop by a café after the seminar?
  This sounds like offering an idea.

• Давай(те) зайдём в кафе после семинара.
  = Let’s stop by a café after the seminar.
  This is more direct.

So:

• Почему бы нам не... = softer, more tentative
• Давай / Давайте... = more direct proposal

Normally, no. In this pattern, не does not make the whole sentence truly negative in the way an English learner might expect.

The sentence is still a positive suggestion:

• Why not stop by a café?

So you should learn Почему бы нам не + infinitive as one expression rather than analyzing не as ordinary negation every time.

A natural pronunciation would usually place stress on the new or important information, often something like:

• Почему бы нам не зайти в кафе после семинара?

Depending on context, a speaker might stress:

• кафе if the place is the main idea
• после семинара if the timing matters
• нам if contrasting with someone else

In normal conversation, the whole sentence would sound smooth and inviting, with rising intonation at the end because it is phrased as a question/suggestion.

A useful template is:

• Почему бы + dative pronoun/noun + не + infinitive?

Examples:

• Почему бы мне не позвонить ему?
  = Why don’t I call him?

• Почему бы тебе не отдохнуть?
  = Why don’t you rest?

• Почему бы нам не пообедать вместе?
  = Why don’t we have lunch together?

So your sentence fits this exact model:

• Почему бы нам не зайти в кафе после семинара?

A good English shortcut is:
Почему бы нам не... = Why don’t we...? / Why not...?