What part of speech is tied down?

Answer 1

Tied down is a phrasal verb, as in "He tied down the load and drove away." Tied down is the past tense of tie down.

Tied is the past tense or past participle of the verb tie. Down is an adverb.

Answer 2

"Tied" is a verb in the past participle form, while "down" is an adverb particle in this context explaining the action of tying. Together, they form a phrasal verb "tied down" which means to secure something firmly in place.