Actually, I've asked people who speak Hebrew, and they can read the OT, but they would rather read it in Modern Hebrew.
And English-speakers can read Shakespeare but they'd rather read Reader's Digest
Many of the grammatical tenses are simplified. I don't know all the details.
Essentially, where Biblical Hebrew had aspect, Modern Hebrew now has tense. The perfect has been reanalysed as a past tense and the imperfect as a future, with the former present participle supplying the present tense. Shifts like this aren't unusual; aspect was more prominent than tense in Ancient Greek as well, but nowadays the past/non-past contrast is more salient (as is the case in neighbouring languages as well). Since all Mediaeval Hebrew speakers were bilingual in other languages, most of which used tense-prominent systems, this shift may well go back centuries.
I also know that Jews spoke Hebrew only for religious services in the 1800s.
Speaking is not the only measure of the vitality of a language. The Jews continued to write poetry in Hebrew from ancient times to the present day. Under the influence of the 18th-century Haskalah
movement, they began using it write about a wide variety of non-religious subjects including politics and culture. Even before then, tracts on philosophy, medicine, philology, and so forth were written in Hebrew. This is the reason why when Ben Yehuda went about fashioning Hebrew vocabulary for modern life, he didn't have to start from scratch. He had centuries worth of coinages to draw upon already.
Hebrew was for religion, Yiddish was for every day talk.
Only in parts of Central and Eastern Europe. Keep in mind that there were Jews in every nation of Europe by that time, not to mention throughout North Africa, the Middle East, and as far away as India (the Cochin Jews
) and China (the Kaifeng Jews
). Just as the Christians of mediaeval Europe relied on Latin for intercommunication, Jews from disparate regions wrote and spoke to each other in Hebrew.
Like you said, it was from parts lying around.
No, it was from a coherent whole. Hebrew didn't have to be cobbled together from a dozen different languages; it just had to be recodified.
So was Esperanto for that matter. Esperanto is not an a priori language like Klingon. Hebrew was not spoken as a vernacular until Israel was created and they chose Ben Yehuda's retooling of Ancient Hebrew.
The use of Hebrew as a vernacular predates the creation of the State of Israel by at least thirty years.