The way the "if" clause is used indicates that the stated situation is TRUE at the present time. Hence, it refers to a specific illness, in this case, it is THE FLU. It you want to use "an Influenza", then you can write the "if" sentence structure as NOT true for the present time as:
If you have an influenza, you should see a doctor.
In the above sentence, the situation stated is NOT true at the present time, it is just a guess, so it is NOT specific.
The children have been enjoying themselves today.
The children enjoyed themselves today.
The major difference between a "past tense" and a "present perfect progressive tense" is the duration of the event. For "past tense" is very short, yet "present perfect progressive tense" emphasizes continuity of the event. Hence, you can use both of them in the above sentence, it is just how long the event lasts that you want to express. However, based upon the written sentence, it is better to use "present perfect progressive tense", because you want to emphasize "children have the fun for a long while".
Again, you can use both "as for" and "until" in this sentence. However, you need to know the difference between them.
To use "as for Friday", you ONLY care about "Friday". What has happened before Friday, it is NOT your concern.
To use "until Friday", you DO care about what has happened before Friday. In fact, you emphasize "he has been working on the report, but he has not finished".
I can't understand you when you speak while chewing your food.
This is a complex sentence (複合句). You can have more than one subordinate clause (附屬子句) . In this sentence "when you speak" is an object complement (受詞補語) to describe "you". "while chewing your food" is a V-ing phrase (動名詞片語) uses as an adverb to describe "speak".
If you have difficulty to understand a complex sentence structure, you can think it as:
I can't understand you when you speak and chew your food.
2012-02-17 03:41:54 補充：
I have difficulty to type Chinese, so if any Chinese is NOT correct, please feel free to tell me.
2012-02-17 22:06:34 補充：
Master Louis! Thank you for reminding me about uncountability of "influenza".
In this case. The sentence in the answer area needs to change to:
If you have influenza, you should see a doctor.