Can one marry a woman who is still in her ʿiddah?

It is forbidden and invalid to marry a woman in her ʿiddah, whether the ʿiddah of talaq or the ʿiddah of her husband's death.
Allah says: "And do not resolve on the marriage contract until the prescribed term is fulfilled." Even an explicit proposal to a woman in the ʿiddah of a revocable (raj'i) talaq is forbidden; for the ʿiddah of death or an irrevocable (ba'in) talaq, only an indirect hint is permitted. A contract made during the ʿiddah requires the couple to be separated, and if they have been intimate, the woman must complete her ʿiddah. See ʿIddah & reconciliation.