1. In that or this place, time, or thing.

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

• trein,
• thereoy,
• theron,
• thering,
• terrein,
• theroie,
• thereis,
• themeing,
• thrain,
• theen,
• therteen,
• therei,
• therecan,
• theran,
• therie,
• theten,
• threir,
• theree,
• thern,
• theri

• thorium,
• thorn,
• Thran,
• thorny,
• Theron,
• throne,
• throw-in,
• thrown,
• therrien,
• theorem,
• Thorne,
• thrum,
• thereon,
• therm,
• throw in,
• thoren.

The adjective form of the word "therein" is "therein". It functions as an attributive adjective to describe something that is located or contained in a place previously mentioned.

The plural form of the word "therein" is "thereins".

The singular form of the word "therein" is "therein". "Therein" is an adverb and does not have a specific singular form. It is used to refer to a place or point already mentioned or inferred.