This is my question.Imagine I am going for work now at 8.00 a.m by a car.On the way to work,the street is busy and jammed wih cars.But however I managed to go to work.So after I finished my work,now I am on the way to go back home.But,the street is still busy and the time is 12.00p.m.And I said " Oh my god,what happened/happens/happen to the street?".So my question is which one is correct?Happen/Happens/Happened?.Should I past tense,present or what?Because the traffic jam starts at 8.00a.m and it is still jammed until 12.00p.m.So am I supposed to use past tense or present??By thw way,if there is any grammar mistakes such as using wrong tenses,like past tense and present tense in my whole sentence,please correct me so that I can improve my English.Just point out the mistakes for me so that I can correct it.Thanks
What has happened to the streets?
I'd use "has happened". But let me rewrite your text in the form of a narrative (I used only the simple present tense (active and passive). The problem with your version is that you mix tenses too much.

At 8 am I leave for work by car. On the way to work the street (I suppose the listener/reader knows what street you're talking about) is jammed with cars. Somehow I manage to get to work. After work I'm on my way home but the street is still congested at 12 pm. And I say "Oh my God, what's happened to the street".

Now, I'm not sure how natural it is to say "what's happened to the street" but I suppose it's possible. Offhand I can't think of anything better.
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Can I say "What have happened to the streets?".Why you use has?Is it What always followed by singular verb?Example,'What makes you handsome','What has happened to him',.Thanks
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The happening is presumed to be singular: one thing has happened to all the streets.
I'd say:

what has happened?


what is happenning here?