Learn Croatian: Pao sam s bicikla i trebam dezinficirati ogrebotinu. - I fell off the bicycle and need to disinfect the scratch.

In Croatian, prepositions of movement work in pairs based on where you were to begin with. Because you ride on a bike (na biciklu), you must use the opposite of na when leaving or falling off it, which is s (or sa). If you were inside a car (u autu), you would use iz to express getting out.

Why is it s bicikla and not sa bicikla?

You generally only use the longer form sa if the following word starts with a sibilant sound (like s, z, š, ž) or a difficult consonant cluster (like sa mnom). Since bicikl starts with the letter 'b', the short form s is used because it is easy to pronounce together.

Is the word for bicycle feminine since it ends in 'a' here (bicikla)?

No, the standard dictionary form is the masculine noun bicikl. Here, it is in the genitive case because it follows the preposition s. Note that in some coastal dialects of Croatia, you might hear locals treat it as a feminine noun in everyday speech, but standard Croatian keeps it masculine.

Could I say treba da dezinficiram instead of trebam dezinficirati?