Use can when your sentence is about something someone is genuinely able to do, when you're giving permission or asking for it in a casual setting, or when you're saying something really happens sometimes under a condition. In every one of these situations, can is followed by the base verb, never to: I can speak English. That's what you'd tell an employer about the languages you actually handle. The same pattern holds when you warn a teammate about a possible event, as in The server can crash if traffic spikes. Writing can to speak marks it wrong immediately.
You're telling a colleague what you can handle: I can join the call at noon. You're saying you're able to be there, so can fits, and the verb stays bare after it, never can to join.
Someone asks what you can offer a client: I can prepare the report by Friday. Here can shows what you're genuinely able to do, and the bare verb prepare carries the meaning on its own.
You're naming someone else's ability: My sister can fix bikes. Same rule, can plus the base verb, and the sentence simply reports what she's able to do.
A colleague asks you for leave: Can I leave early today? You're asking for permission in an informal setting, and can takes the bare verb leave. The same shape works when you grant it: You can leave early.
Now the contrasting case: something that genuinely happens sometimes. The server can crash if traffic spikes. You're not saying anyone is able or willing, you're saying it's possible under a condition, and can still takes the bare verb crash.
Notice the shift in who's behind the sentence. In the ability cases, a person's skill or availability is doing the work. In The server can crash, no one chooses anything, the system just sometimes fails, and the if clause states when it happens. In permission uses, the choice belongs to whoever grants it.