Correct spelling, explanation: the base form of maintained is maintain, which is a regular English verb. Maintained derives from the Latin expression manu tenere, which could be translated as hold in the hand. Later, maintained was also affected by the Old French word maintenir and finally, in the Middle Ages, maintained entered the English language.

Definition of maintained:
1. verb – to keep something at the same level, to continue something,
For years the company maintained the highest standards, but once the SEO quit, everything changed.
2. verb – to look after something so that it stays in a good condition,
It’s a complicated machine and Jack maintained it really well so it’s a huge shame that he left the company.
3. verb – to believe strongly that something is true,
He maintained that he had nothing in common with that woman while I’m pretty sure that I saw him with her in the bar a week ago.

Expressions with maintained:
A few common expressions with maintained are: maintained control, maintained order, and maintained peace.
The team from England maintained control for most of the game, but they lost a point in the last minute of the game.
I really liked our previous boss, maybe he was demanding, but at least he maintained order.
He is considered to be one of the most responsible kings of this country as he maintained peace for almost fifty years, which was rather uncommon for those uneasy times.

Incorrect spelling, explanation: maintain and maintenance are two similar words, which are often confused by English users. The former one is a verb and its past form is maintained while the latter one is an uncountable noun and nothing can be added to it. However, instead of using the form maintained, users of English think that maintenance is a verb and a common mistake is a form maintenanced, which is wrong.