The Bible was originally written in three different languages. In the OT, it was predominantly Hebrew with a brief section of Aramaic and a scattering of proper names and words adopted from other languages, like Babylonian and Assyrian. In the NT, it is almost entirely Greek, but again there is a brief scattering of proper names and places from Aramaic, transliterated Hebrew and Latin. Hebrew and Aramaic, as languages, are quite different from modern English.
OT Hebrew, in particular, was written very differently from modern English. Among other things, Hebrew is a right-to-left language, so the reading order is reversed, and the written construction of words is different. OT Hebrew does not possess any punctuation, like the modern period or comma, so you will read a LOT of conjunctions ("and he walked here and he walked there and he said I am tired of going here and I am going to lie down now and when I get up..."). This is intentional, because conjunctions were really the only way they had to denote the end and beginning of different sentences in a written medium. Similarly, OT Hebrew does not contain any difference between upper and lower casing. Everything is written in the same case. This means that translators have to guess what words are proper names and what words are descriptive. This is especially challenging when words are both used as a generic noun and as a place name. The best example of this is Adam, which is literally the Hebrew word for "man". So it's unclear if the "Adam" written in Genesis 1-3 was meant to be interpreted as "the man blah blah blah" or "Adam blah blah blah". The way Hebrew is written is that the consonants are characters and the vowels are special markings on those characters. Unfortunately, OT Hebrew omitted all of the vowel markings. I don't know why, but they did. This unfortunately leaves a lot of ambiguity in the pronunciation and even interpretation of numerous passages in the OT. Modern scholarship has managed to nail down most issues resulting from this ambiguity, but there are still a few important cases left. The biggest and most well-known of these issues is the pronunciation of YHWH, the Tetragrammaton, which is referenced in the OT as the name of God himself. Yikes. So it should be apparent why a lot of people want to get it right. Unfortunately, just as with every other word in the OT, there are no vowel markings, so we pretty much have to guess. Some people use different markings to achieve the words Jehovah or Yahweh. Those are the most popular pronunciations, but both of them are simply creative guesses. We don't actually know. Lastly, OT Hebrew is thousands of years old and the OT contains a lot of obscure words for which we do not have any proper translation. The translators simply have to guess at the meaning of these words in the context of the passage, which accordingly results in differences between some versions.
The Greek NT is much more closely aligned with modern English in the sense that 1) it has vowels, 2) the verbal tenses and conjugation are almost (but not quite) the same as English, 3) the language is much more recent, so our understanding of the vocabulary is comparatively better. Some difficulties remain however. First, there are still words for which we don't have proper translations (or at least debatable translations). Second, NT Greek does not have casing, just like OT Hebrew, so it is ambiguous in some places what are proper nouns and what are generic nouns, just like in the OT Hebrew. Third, NT Greek doesn't have any punctuation, so again the translators of the Bible have to guess when one sentence ends and another begins. None of these modern conveniences existed for ancient authors or ancient readers. They probably walked uphill in the snow both ways, too. :) My understanding is that NT Greek does put spaces between distinct words though, so that is useful. For reference, Latin scripts from that time period do not use spacing, so every word flows directly into the next. Maybe that's why literacy rates are so much higher these days?