Prefrontal Cortex-where is it and why does it matter?

Prefrontal cortex is the subdivision of frontal lobes. Basically as the name tells us, they are on the front area of fronal cortex, which is around the above of our eyes. The frontal lobes include the important mortor cortex (which affects our somatic senses), and the famus "broca's area" which is thought by some linguists as "main language area".Compared to other mammals like cats, dogs, or even monkeys, we human beings have the most extended area (the biggest) of prefrontal cortex.

The part that's right above our eyes, (dorsal lateral prefrontal cortex, DLPFC) is thought to be important for "working memory," -things that can be held for a very short period of time. (dude, i feel this term takes another paper to explain the definition).

The developing and existence of this part of brain function can be seen on experiments done on infants, young children, and monkeys (whoes DLPFC are knocked out).
For example, the A-not-B experiment:
There are two wells in front of a subject (a monkey with DLPFC being removed or a cute baby). The subject watches a reward being put into one of them (say A). Then the wells are covered, a curtain might fall to block the vision and causes delay. Finally the subjects can reach to the well where they think the treat is.

Both the monkey and human baby would reach to A. Then they get the reward.
However, the next time, they would watch a reward being put in well B. This time, given the location they have already seen, they would still turn to the successful pattern of reaching to well A.

Another test they do on younger kids is to ask them to shout out "day!" when they see a card of moon and star, and "night" when they see a card of a sun. Children who were bellow 3 did a poor jobs than older kids. Even though they showed understanding of the rules.

In order to acoomplish the tasks, there must be some information being held active at hand (e.g. remembering the location of the treat, or remembering the rule of the test). But there's also another piece of information coming in/being held that are competing with the other (e.g. seeing the card change, remembering the successful, conditioned location of the treat). An adult or normal person w/o brain damage to DLPFC can inhibit one and reach to the other. DLPFC seems to be in important position in this "inhibitory" function of working memory.

It's actually a good news for us being human. That means we are not just reponsive to any stimulus, or old patterns. We can integrate the new information with the old data, and then make the best solution. Otherwise we would never have new ways of dealing things, like some brain damage patients, who kept pressing the same button or stick onto one way of sorting cards, despite the inspector told them the rule had changed.

Regarding the latency of 3 year old children in the "day-night" test (given that they know the rule correctly)--It's more of the cognitive difficulty. What about learning a language? Normal adults are assumed to have normal cognition. But sometimes I found that, although I know the grammar (maybe not perfectly, but pretty well), the setence just won't come out right. There might be some coordination problem, since motor cortex, especialy for the tongue, and mouth is close to this area. But what's the relationship of working memory, DLPFC, and language acquisition? We can’t be all brain-damaged for learning a language (or are we?)