Sour
い adjective
The taste of something with acid is... well... you'd think it'd be acidic, but it's a sour taste. Some acidic things are pretty sour, though. Lemons are pretty acidic, and they're super sour too, so think about that when you need to remember this word.
Something was so sour that it broke your mouth. You were so upset about this, that you decided to sue (す) the maker of this nasty sour thing.
最初に食べた食べ物は腐りかけていて酸っぱかったです。
The first food I ate was rotten and sour.