I downloaded several movies in MKV format, but on my Mac they either won't open or constantly freeze. Is there a better player?

This is a very common problem because standard macOS players often don't support modern formats such as MKV or FLAC. I recommend trying Elmedia Video Player, which can be found at https://www.elmedia-video-player.com/ — it's a universal player with support for various formats and stable streaming to other devices. It does not require additional codecs, allows you to control subtitles, playback speed, and even stream videos to Smart TV. I have been using it for a year now, and it is much more convenient than standard programs. In addition, the player has a modern interface and does not overload the system, which is especially important for older Macs.