I recently heard this at a halacha class in the name of Rav Kanievsky:
Because people sometimes put the accent on the wrong part of words, theoretically, a person could go their entire life without every really saying krias Shema, ch'v! (that's the R' Kanievsky part)
What matters in krias Shema and krias HaTorah is if the wrong accent or wrong vowel changes the meaning of the word.
Sometimes the meaning is not changed, in which case interrupting the Torah reader or repeating verses that you recited inaccurately is unnecessary. However, sometimes the simplest mis-accent can change the meaning entirely.
Read the line below out loud or in your head and notice where you put the accents.
V'ahavta et Ado-nai Elo-hecha...v'shinantam l'vanecha, v'dibarta bam...
Now which of the two accent versions below match what you said aloud?
V'ahavtA et Ado-nai Elo-hecha...v'shinantAm l'vanecha, v'dibartA bam...
V'ahAvta et Ado-nai Elo-hecha...v'shinAntam l'vanecha, v'dibArta bam...
If you said the first version (V'ahavtA), you are saying these words correctly. If you said the second version (V'ahAvta) you are saying it incorrectly. This is confirmed by the placement of the cantillation trope on those words.
Does a Little Accent Really Matter?
Yes. The meaning of the whole sentence changes!
V'ahavtA is the command form, "love" - "thou shalt love Hashem"
V'ahAvta is the past tense form, "you loved" - "and you loved Hashem"
The above two versions translated:
V'ahavtA et Ado.nai Elo.hecha...v'shinantAm l'vanecha, v'dibartA bam...
"Love Hashem your G-d...and teach them to your children, and speak of them...
"V'ahAvta et Ado.nai Elo.hecha...v'shinAntam l'vanecha, v'dibArta bam...
"You loved Hashem your G-d...and taught them to your children, and spoke of them..."
Proper Krias Shema
Generally, if you read the Shema with the cantillation trope, you will be forced by the trope to put the accent in the correct part of the word. Even if you don't know what the trope sounds like, you may be able to still use the trope notes as a visual guide for accenting.
You can also just remember that the command form of a verb usually places the accent on the last syllable, whereas the past tense form of a verb places the accent on the second to last syllable.
"hevei zahir bekalah kebechamurah she- ein atah yodea mattan secharan shel mitzvot."
Be as scrupulous about lighter commandments as you are about the most stringent norms for one cannot know the corresponding reward of different commandments.