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profitendieu did a really good job describing the main translation of 本来. The best translation is usually "originally" or "from the start".

Sometimes, it can also translate to "of course". For example:
本来就该这样办。 = Of course it should be handled that way.

Also, sometimes it just translates to past tense in English. For example:
他们应该更加重视质量。- They should pay more attention to quality
他们本来应该更加重视质量。- They should have paid more attention to quality

If it is available to you, jukuu.com (句酷) is a helpful website for me. It is a website designed to help Chinese speakers learn English (but I use it the other way). You can search for a word or phrase in either English or Chinese and it will give you example sentences in both Chinese and English, so you can see how the word is translated or used. (That's where I got my example sentences from)

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本來 generally translates to phrases like “originally," “originally planned to,” "was supposed to," "used to," etc. The exact translation depends on the circumstances.
My Chinese is very bad, so I'm not completely sure if these sentences are idiomatic, but here are some examples:
- 我本來今天上學，但是我早上發燒。I was (supposed/going) to go to school today, but I had a fever this morning [so I couldn't go].
- 本來他有五個蘋果，現在有兩個。他吃了多少蘋果？Before [=originally], he had 5 apples, and now he has two. How many apples did he eat?