In the Spanish language, why is the accent eliminated when a sentence is composed into the plural tense?
la lección becomes
Notice how 'ó' becomes 'o' (without accent).
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Perhaps you are used to languages where the accent changes the sound but in Spanish it indicates stress (and a few other things which need not concern us here).
In the singular the accent is needed because the final syllable is stressed and according to the normal rules as a syllable ending in «n» it should be.
When you add the suffix «es» for the plural, the stressed syllable is no longer the last one and so it is required NOT to mark it. It is still stressed though.