Starting a full sentence with those two words is pretty much guaranteed to get your knickers in a twist, grammatically.
It happens because Dutch sentences can easily begin that way, with alleen als followed first by the conditional part and then the result. The natural structure in English splits the only from the if, so you only get the result if the condition is met. It’s probably clearer with an example or two:
- Only if it’s a Friday stay the shops open late
=> The shops only stay open late if it’s a Friday
- Only if the egg cell is fertilized does the ovule grow into a seed
=> The ovule only grows into a seed if the egg cell is fertilized
Note that I specified a full sentence, by the way. Starting a partial sentence as a reply in speech with those two words is commonplace, for instance:
“Are you going to the party?”
“Only if Julie’s going to be there.”
Prevalence: high. A general point that can occur in pretty much any kind of source text.
Frequency: high. Surprisingly widespread.
Native: no. We’re not mentally wrestling with Dutch sentence structures.