For words like forum and datum, both are possible, but also there's a clearly more commonly used option that doesn't confuse everyone. In Dutch, fora is probably about equally common as forums; in English I'm not sure I've seen it ever used. It's like using datum to mean a single data point: "look at me being clever about word technicalities and making everyone do a double take to understand this sentence". At least, that's how I feel when seeing a native speaker use datum instead of data point in English, or in Dutch using data to mean multiple calendar dates (the common word being datums).
I don’t know when I internalized that “data” is plural and its singular is “datum”, but it wasn’t when I had a similar reaction to “fora”. The former is just how my brain handles the word’s count, and the latter is very much like your reaction.
