Depressing moment looking at some students doing review problem sets for me.... 100% of my students can tell me that "te quiero" means "I love you." I'm pretty sure, also, that they could tell you that "te" means "you" in that sentence, and that the "o" ending there means "I."
However, applying this same pattern to any other sentence (any sentence not already memorized as a stock phrase) is shockingly hard for even bright kids to do consistently.
For example, if I give a test with "la quiero" or "me dan una coca cola", a good percentage of students will answer "she wants" or "I give them a soda."
I've hammered on this fundamental bit of Spanish just about every way I can think of, yet I still can't say I've made half as much progress with this as I'd like to. It seems to take well over a year for this pattern to really sink in... if I weren't speaking from experience, I'd think it could be done with one day's lesson.