A subordinate clause beginning with "if"
Should this sentence have a comma: "It was dark, if you recall." It's a dependent clause so it should not need one, but it doesn't sound right without it.
In a comment, Edwin Ashworth wrote:
The clause 'if you recall' here is a pragmatic marker soliciting confirmation (cf 'wasn't it?') or encouraging aquiescence (cf 'as I'm sure you'll agree'). It stands apart from the statement in the main clause. I'd certainly add the comma. The situation is different with say 'It was dark if clouds heralding one of the frequent storms came close'.