Shouldn't that be "שתים" because the vowel is "e", not "ayi"?
In the number twelve שְׁתַּ֫יִם is shortend to שְׁתֵּים, where the Jodh is only a mater lectionis for the Tzere, not longer a consonant. Therefore you can write in the full spelling too only one Jodh.
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Why is it לימוד and not למידה?
Well, לְמִידָה and לִמּוּד mean mostly the same thing, being action nouns of לָמַד learn and לִמֵּד teach, after לִמּוּד shifted in meaning to studying, learning.
Thank you IngeborgHa14.
Fantastic analysis, Ingeborg! So, how do you say "teaching" (the verbal noun)?
Well, how about the Hiph'il הוֹרָה teach, instruct (root ירה), whose verbal noun is הוֹרָאָה teaching, instruction, pedagogy?
Why is היא not acceptable here instead of זה
שתיים עשרה שעות לימוד ביום היא יותר מדי
Well, שָׁעוֹת hours is plural, and the copula הוּא must agree in number and gender.
how does זה agree in number and gender with "hours"?
Well, it does not. In sentences like a X is (by definition) a Y, זֶה tends to be uninflected: הֵרָיוֹן יֶה לֹא מַחֲלָה pregnancy is not a disease or דִּבּוּרִים זֶה לֹא מַעֲשׂוֹת words are not deeds. The rules for the copula הִיא are different.