Well, it should be underst oo d in all sentences I don't think there's a correct or non-correct one; their use depends on the context. To me the first one sounds literary, the second one formal, and the third one "everyday English"
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CliveYou could also say, of course, the very common form, Now I understand the sentence.Why you don't put the comma after the adverb `now'? Because I tend to do it.