Learn Spanish (Spain): Cuando llueve, cojo el autobús.

The infinitive is coger. If we kept the 'g' and wrote cogo, it would be pronounced with a hard 'g' sound (like in the English word 'go'). Changing it to a 'j' in cojo keeps the same soft consonant sound as the infinitive. This spelling change only happens in the 'yo' (I) form of the present tense.

I have heard that the verb coger is a bad word. Is it safe to use here?

Yes! In Spain, coger is the standard, everyday verb for catching a bus, train, or taxi. However, in many parts of Latin America, it is highly offensive slang, and they use tomar instead. Since this is Spanish from Spain, coger el autobús is perfectly normal and polite.

What is the infinitive of llueve and why does it look different?

The infinitive is llover (to rain). It is a stem-changing verb, meaning the 'o' in the root changes to 'ue' when conjugated. Notice that Spanish does not use a pronoun for 'it' here; the verb llueve means 'it rains' all by itself.

Why do we use the normal present tense llueve after cuando?

Because this describes a habit or a general fact (every time it rains, I do this). You use the normal present tense for regular routines. You will later learn to use a different verb form (the subjunctive) after cuando, but only when talking about a specific future event, like 'When it rains tomorrow...'.