Basic phrases

ja - yes
nej - no

tack - please (used at the end of a sentence); thanks, thank you
tack så mycket - thank you very much
ingen orsak - you're welcome

hej or hej hej - hi, hello
hejsan - hi (informal)

god dag - hello; goodbye (literally: good day; can be used any time from morning until around 5pm)

god morgon - good morning (used until around 11am)
god förmiddag - good morning (used from around 11am until noon)
god middag - good afternoon (used around noon)
god eftermiddag - good afternoon (used from noon until around 5pm)
god kväll - good evening (used from around 5pm)

välkommen! - welcome!
välkommen till ... - welcome to ...

hej or hej hej - bye
hejdå - goodbye
god natt - goodnight
vi ses! - see you! (informal)
vi ses snart! - see you soon!
vi ses senare! - see you later!
ha en bra dag! - have a good day!

ursäkta mig - excuse me (to get someone's attention or to get past someone); sorry
förlåt - sorry
inga problem - no problem
det gör inget - don't worry about it