A useful recognition pattern is: لَا + an indefinite noun without tanwin in the accusative form. Then identify what completes the information as the predicate.
Check
In the example
Result
Is the noun indefinite?
رَجُلَ
yes
Is it without tanwin?
رَجُلَ, not رَجُلًا
yes
Is its accusative form visible?
final fatḥah
yes
Does a predicate complete the meaning?
قَائِمٌ
yes
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Negating an Entire Class with لَا
A useful recognition pattern is: لَا + an indefinite noun without tanwin in the accusative form. Then identify what completes the information as the predicate.
Check
In the example
Result
Is the noun indefinite?
رَجُلَ
yes
Is it without tanwin?
رَجُلَ, not رَجُلًا
yes
Is its accusative form visible?
final fatḥah
yes
Does a predicate complete the meaning?
قَائِمٌ
yes