(1) Kalimat ini bagus = This sentence is good.
(2) Ini kalimat yang bagus = This is a good sentence.

'ini' marks the end of a noun phrase. (see first sentence).
(kalimat ini = this sentence)

In the second example, 'ini' starts the sentence.
'ini' is a sort of a one-word-noun-phrase.
'ini' is the subject of the sentence.
Such a sentence is translated as "This is ..."

The same applies for 'itu'.