Flashcards are one of the oldest language-study tools, and also one of the most commonly used badly. The difference between flashcards that work and flashcards that waste time comes down to a few specific design choices.
A card that shows a word in your native language and asks you to produce the target-language word (recall) builds a much stronger memory than a card that shows the target-language word and asks you to merely recognize its meaning. Recognition feels easier and gives a false sense of mastery — recall is what you actually need in real conversation, so test in that direction whenever possible.
A card that asks you to remember a word's meaning, gender, and an example sentence all at once is testing three things poorly rather than one thing well. Split multi-part information into separate cards (or make the extra information a hint, not part of what's being tested) so you get clear, specific feedback on what you actually know.
A card built around a full sentence gives the word context and grammar exposure at the same time, and tends to produce more usable memory than an isolated word-pair card — see How Memory Improves Language Learning for why context-rich encoding helps retention.
A paper flashcard deck reviewed front-to-back treats every card equally, regardless of how well you know it. A spaced-repetition system tracks your performance per card and schedules easy cards further apart and hard cards sooner — the same total review time produces noticeably better retention when it's allocated this way instead of flatly.
See also the reference page: Why Spaced Repetition Works.
For most learning goals, showing the native-language word and asking you to produce the target-language word (recall) builds stronger memory than showing the target word and just recognizing its meaning.
Generally no — testing multiple facts on one card gives unclear feedback about what you actually know. Splitting them into separate cards, or treating extra details as a hint rather than the tested fact, works better.
Often yes — a sentence gives the word context and grammar exposure together, which tends to produce more usable memory than an isolated word pair.
A plain deck reviewed start to finish treats every card equally regardless of how well you know it. A spaced-repetition system schedules review per card based on your actual performance, concentrating time where it's genuinely needed.