At this time, most children are fluent speakers and can get through most expressions without a mistake. However, they can go wrong when trying out new expressions. Children now mix different constructions (quite a very difficult one).
They use prepositions I'm bored at shopping and put weak endings on strong verbs (tooken instead of taken).
These mistakes are made till about 10 or 11 years but the mistakes are fewer.
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From the 5th year onwards there is an increase in fluency. By 4 and a half most children are composing sophisticated, complex sentences with the help subordinating conjunctions. If, what, when, because, that, where, before (since doesn't come till the sixth year).
There are very few connecting words and phrases that are normally used in adult conversation that make sentence transitions sound easy and natural: well, for instance, like, say, instead and anyhow. Only when children use words like this to link their sentences into a continuous sequence will childrens speech start to sound like that of an adult. No such linking words are found in most 4 - 6 year olds.
A survey by Crystal found that linking words begin in year 7 (age 6 - 7/8) and increase in number after 8 years old but 12 year olds use far fewer linking words than adults and words they used were still simple: now, then, so, anyway, though and really.
The distinguishing marker between adult and childish speech is not the ability to talk fluently (five year olds are already fluent talkers) but the ability to link sentences into smoothly-flowing, natural sounding sequences with the help of a wide range of linking words.