Keeping my instrument in tune is one of the most difficult things I've learned. And I didn't start getting good at it until I owned a tuner. It is essential to use a tuner when you are learning a stringed instrument. Strings are a complicated thing to advise on. Some like 'em thick and weighty, others like noodles.
Strings also lose tone.
I'm not sure that I understand it completely. For sure, the rust on the string inhibits vibration and effects the tone. But there is something else as well. When I say crimp, I mean a little dent in the string. When the dent straddles the nut, the string will be just a little flat.