Within a few months of producing one word utterances children will begin to produce two-word phrases. The two-word stage often occurs from 18-24 months, consisting of utterances generally two nouns or a noun and a verb. Some examples of this include baby chair meaning 'The baby is sitting on the chair' or doggie bark, meaning 'The dog is barking'.
During this stage it is common to see the appearance of single modifiers, for example "That dog", two word questions such as "Daddy eat?" and the addition of the suffix - ing into words to describe something that is happening, for example "Doggy sleeping", instead of "Doggy sleep".
Late on in the two-word stage generally consist of grammatically correct sentences, but will need much guidance from you, the parents. An essential aspect of child language acquisition is learning from a parent, providing guidance to your children.
Confusion of what a child is saying is common during this stage, because the child does not know what plurals and tenses are yet.
Although not in English, the video above identifies the two-word stage of child language acquisition, showing the use of two word utterances.