It is said that the biggest pronunciation problem for Spanish speakers is differentiating between between short and long vowels. Spanish and English each have five vowel letters, but the similarities stop there.
English uses the five letters a, e, i, o, u, to make 12 vowel sounds — those heard in beet, bit, bait, bet, bat, bot, bought, boat, book, boot, butt, and the second syllable of chocolate.
Spanish, however, only has five vowel sounds, one per vowel letter, as heard in para “for,” pera “pear,” pira “pyre,” pora “leek,” and pura “pure.” (These correspond to the five vowels of bot, bait, beet, boat, and boot.) A few words in Spanish even manage to use all five vowels! Eg. abuelito “grandfather” and murciélago “bat.”
Spanish learners may have a very difficult time producing or even perceiving the various English vowel sounds. Specific problems include the inability to distinguish between the sounds in words such as ship/sheep, taught/tot, pool/pull or cart/cat/cut.
It's impertant to focus on these small variations if you'd like your students to achieve a higher level of fluency. Accents are lovely, but when pronunciation errors cause confusion or an iniability to communicate effectively, they must be a priority.
Enjoy your weekend and happy pronunciation!