1. Into which; in which thing, time, respect, book, &c.; in what.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• wheren,
• whern,
• whitein,
• whereup,
• wherent,
• hereing,
• whren,
• wherea,
• wereing,
• wherin,
• whering,
• wheezin,
• wheree,
• whenin,
• wherehe,
• therin,
• whereva,
• wheen,
• werein,
• wherer

• whereon.

The adjective form of "wherein" is "wherein" itself. It functions as both an adverb and a conjunction.

The plural form of the word "wherein" is "whereins".

The singular form of the word "wherein" is "wherein". The word "wherein" is already in its singular form. It is used to refer to a specific location or circumstance in which something happens or exists.