A Word of Advice
Један савет — Recommend, warn and soften what you think someone should do.
A neutral recommendation
«Требало би да» + present is the everyday way to say should: «Требало би да одмориш.» The form does not change with the person.
Требало би да разговараш с њом.
You should talk to her.
If I were you
«На твом месту бих…» makes advice personal but not commanding. The participle agrees with the speaker: рекао бих / рекла бих.
На твом месту бих сачекао.
In your place, I would wait. (male speaker)
A gentle nudge
An imperative can sound friendly with покушај or немој да: «Покушај да легнеш раније.» «Немој да журиш.» Formal or plural forms are покушајте and немојте.
Покушај да му мирно објасниш.
Try to explain it to him calmly.
Better and advisable
«Боље је да…» and the slightly more formal «Ваљало би да…» present an option without ordering the listener.
Боље је да кренемо сада.
It is better for us to leave now.
Advice is not obligation
«Требало би» recommends; «мораш» imposes a necessity. Soften further with можда or мислим да. Keep clitics early: «Можда би му требало рећи.»
Мислим да би требало да провериш.
I think you should check.
Put It Together
Keep the lesson goal in view: Recommend, warn and soften what you think someone should do. Revisit the examples, then test yourself in practice.