the single and plural forms of debris

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There is no plural, and the word is always pronounced without the /s/. It is an uncountable noun: There is not much debris remaining from the 9/11 attack.
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Math Entry: debris
Pronunciation: de-’brê, dã-’, ‘dã-1, Brit usa ‘de-(,)brè
Function: noun
Inflected Form: plural debris \-‘brez, -,brez\
Etymology: French debris, from Middle French, from debriser to break to pieces, from Old French debrisier, from de- + brisier to break, of Celtic origin; akin to Old Irish b

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I doubt that M-W has made an outright error-- no doubt they found someone using it in a plural forum. My Shorter Oxford does not indicate any plural, and all of the other major on-line dictionaries characterize the noun as uncountable. Note the on-line Oxfords' definitions:
debris: noun 1 scattered items or pieces of rubbish. 2 loose brok

I dont know I am trying to figure out forms of the word debris, but every website I have tried doesnt work. Do you know anything that couild help me? O and by the way, here is a sentence using the word debris: After the tornado, we saw lots of debris and we picked it up.
Thats how you can use the word.

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[url=http://dictionary.cambridge.org/define.asp?dict=CALD&key=19984&ph=on]DEBRIS[/url] is an uncountable noun: Much debris; little debris.
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Any uncountable noun can be made plural, provided that they are used properly. For instance:

"The waters at the surface of the North Atlantic Ocean are saltier than those of any other
ocean."

"In deeper waters, the North Atlantic temperatures can vary from about 2.5°C at the very bottom to about 5°C at 3,000 feet."

"Worldwide, the waters around the
equator are

Can both of these be correct.
1. Is there any excess visible debris in the oil filter?
2. Are there any excess visible debris in the oil filter?

No. Read my earlier post.

Always singular, no matter what!