How to translate 就 into English as in 这就是个很好的例子?
就 http://www.cantonese.sheik.co.uk/dictionary/characters/808/


就 has many usages,  I would say "definitely" is applicable here:

这是个很好的例子 - This is a very good example

这(就)是个很好的例子 - This (definitely) is a very good example

是他偷了我的钱 - He is the one who stole my money

(就)是他偷了我的钱 - He (definitely) is the one who stole my money

'就' (definitely) here is for emphasis function. Without it, the sentence's meaning doesn't change much, but with it, the emphasis on the object and the tone of seriousness would increase. 

就是他 - you sure it is him

是他 -  you say it is him

Since it is for emphasis, you can use 'exactly' or 'actually instead of 'definitely' in English