Learn Danish: Når vandet koger, bliver det til damp. - When the water boils, it becomes steam.

This is because the water is physically transforming into something else. In Danish, you use bliver til when a subject changes its fundamental nature or turns into a completely different thing, like water into steam. If you were just describing a change in condition or a new profession, like becoming cold or becoming a teacher, you would only use bliver without til.

Why is the word order bliver det and not det bliver in the second half of the sentence?

This is a classic example of the V2 (verb-second) rule. The entire first part of the sentence, Når vandet koger, is a subordinate clause that counts as the first piece of information. To keep the main verb bliver in the second position, it has to swap places with the subject det.

Since the English word 'when' can also be translated as da, could we use da in this sentence?

No, you must use når here. Da is strictly for a single, specific event that happened in the past. Because this sentence describes a general truth or a repeated action, når is the correct word for 'when'.

Why do we use the pronoun det here instead of den?

Det refers back to vandet (the water). Because vand is a neuter noun, which you can tell from the -et ending in its definite form, you must use the neuter pronoun det to say 'it'.