The negative active second-person masculine forms retain singular, dual and plural:
Word / formMeaning / explanation
Singular: لَا تَفْعَلُyou (one man) do not / will not do.
Dual: لَا تَفْعَلَانِyou two men do not / will not do.
Plural: لَا تَفْعَلُونَyou men do not / will not do.
لَا تَفْعَلُ is not the prohibitive 'do not!' form لَا تَفْعَلْ. The former negates a statement and retains the indicative ḍammah; the latter is jussive and used in prohibition.
The negative active second-person feminine forms are:
Word / formMeaning / explanation
Singular: لَا تَفْعَلِينَyou (one woman) do not / will not do.
Dual: لَا تَفْعَلَانِyou two women do not / will not do.
Plural: لَا تَفْعَلْنَyou women do not / will not do.
The singular feminine form لَا تَفْعَلِينَ retains its inflectional nūn in this paradigm. Feminine plural لَا تَفْعَلْنَ instead contains the nūn of feminine plural, which is morphologically different. The entire ending determines the distinction.