1. By design.

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

• intentionallly,
• intentionaly,
• intentenally,
• intentionnaly,
• unitentionally,
• intemationally,
• intentonaly,
• itentionally,
• interntionally,
• intnetionally,
• interntionaly,
• intentionly,
• intentinaly,
• intensionaly,
• intencionally,
• potentionally,
• intentionallity,
• intentinally,
• intentially,
• intenionally

• inattentiveness,
• intensional,
• internationally,
• intentional,
• indentation,
• inattention,
• inattentively,
• inattentive,
• intentionality.

The adjective form of the word "intentionally" is "intentional".

The plural form of the word "intentionally" is "intentionally". This word does not change in its plural form.

The singular form of the word "intentionally" is "intentional"