meanings

Arriver means “to happen” or “to occur.” It points to a single event or result.
Se passer also means “to happen,” but it focuses on how something unfolds or goes.

Arriver can also mean “to arrive” at a place. Se passer never has that meaning.

usage of arriver for events

Use arriver for a specific thing that takes place. The event has a beginning and an end. Often it is sudden or unexpected.

The verb is used impersonally with il: il arrive quelque chose.

Examples:

• Qu’est-ce qui est arrivé ?
  “What happened?”
• Un accident est arrivé sur l’autoroute.
  “An accident happened on the highway.”
• Il lui arrive toujours des choses bizarres.
  “Strange things always happen to him.”
• Rien n’est arrivé. Tout va bien.
  “Nothing happened. Everything is fine.”
• Si quelque chose arrive, appelez-moi.
  “If something happens, call me.”

usage of se passer

Use se passer for a process or a course of events. It describes how things go, not just the fact that something happened.

It works for longer actions, sequences, or ongoing situations.

Examples:

• Comment s’est passé ton examen ?
  “How did your exam go?”
• La réunion s’est bien passée.
  “The meeting went well.”
• Qu’est-ce qui se passe ici ?
  “What is going on here?”
• Rien ne se passe comme prévu.
  “Nothing is happening as planned.”
• Tout s’est passé vite.
  “Everything happened quickly.”

key difference

Arriver asks for the event itself. What took place?
Se passer asks for the quality or flow. How did it go?

Compare:

• Qu’est-il arrivé ? – “What happened?” (tell me the event)
• Comment ça s’est passé ? – “How did it go?” (tell me the details)

Another pair:

• Il est arrivé une chose étrange. – “A strange thing happened.” (the event)
• La chose s’est passée étrangement. – “The thing happened strangely.” (the manner)

Another way to distinguish them:

• Qu’est-ce qui t’est arrivé ? – “What happened to you?” eg if your friend has a black eye, you want to know the single, simple cause.
• Qu’est-ce qui s’est passé ? – “What happened?” eg you want to know the entire narrative of a movie scene you missed.

synonyms

For arriver (event):

• survenir – “to occur” (more formal)
• avoir lieu – “to take place”

For se passer (process):

• se dérouler – “to unfold”
• se produire – “to occur” (can also work for events)

For “to arrive” at a place (not a synonym, but a second meaning of arriver):

• parvenir – “to reach” (formal)
• venir – “to come”

summary

Arriver = a happening, an event. Use it for the fact that something occurred.
Se passer = a going, a process. Use it for how something unfolded.

Do not use se passer for arriving at a place. Do not use arriver to ask “how did it go?”