Learn Swedish: Du måste tänka på att vem som helst kan läsa det där. - You have to consider that anyone can read that.

While någon means someone or anyone in a general sense, adding som helst emphasizes unrestricted choice. In this sentence, vem som helst means absolutely anyone at all, no matter who it is, stressing the lack of privacy.

Why is the preposition på placed right before the att clause?

The phrasal verb here is tänka på, meaning to keep in mind, remember, or consider. Unlike English, which often requires adding extra words like 'about the fact that', Swedish grammar allows you to place an att clause directly after a preposition.

Could I drop på and just say Du måste tänka att?

No, because tänka on its own usually means to think in terms of a cognitive process or having an intention. To mean keep in mind or consider, you must use the specific phrasal verb tänka på.

Why does the sentence end with det där instead of just det?

Using det där specifically translates to 'that' or 'that thing right there'. It strongly points to the specific text, post, or message the person created, whereas just det would simply mean a much weaker 'it'.