Learn Spanish (Spain): Mis amigos han buceado con tiburones dos veces.

We use the compound past tense (han buceado) because the sentence describes a life experience. When we talk about how many times someone has done something in their life up to now, like using dos veces (twice) or nunca (never), we use the present tense of haber plus the past participle.

How do we form the word buceado?

Buceado is the past participle of the verb bucear (to dive or scuba dive). For regular verbs ending in -ar, you drop the ending and add -ado to make this form.

Why do we use veces for twice instead of a word like tiempos?

In Spanish, when you are counting occurrences or instances (like two times or twice), you always use the word vez or its plural veces. The word tiempo refers to time on a clock, duration, or the weather, so saying dos tiempos would be incorrect here.

Why is it con tiburones without los in front of the sharks?

We leave out the word los (the) because the sentence is talking about sharks in general, not a specific group of sharks. If you said con los tiburones, it would mean they dived with a specific group of sharks that the listener already knows about.