Learn Portuguese (Portugal): Caso haja um incêndio, onde está o extintor? - In case there is a fire, where is the fire extinguisher?

The word caso (in case) introduces a hypothetical situation or condition, which forces the following verb into the present subjunctive mood. Haja is the present subjunctive form of haver (there to be / to exist).

Is haja an irregular verb form?

Yes, highly irregular! While the normal indicative form for 'there is' is just há, the present subjunctive form becomes haja. Because it acts as an impersonal verb here, it doesn't change to plural even if the noun following it is plural (for example, you would still say caso haja problemas).

If I want to say 'If there is a fire', can I just swap caso for se?

You can change the word, but it changes the grammar rules! The conjunction caso takes the present subjunctive. However, if you use se (if) to talk about a possible future event, it triggers the future subjunctive. So the sentence would have to change to: Se houver um incêndio, onde está o extintor?

Could I use the word fogo instead of incêndio?