This is the 194th most frequent Hungarian word.
múlt
"Past"
In this sentence, 'múlt' functions as an adjective describing 'héten' to specify the time being referred to, 'last week'.
A múlt héten sokat tanultam.
Last week, I studied a lot.
Here, 'múlt' is a noun and refers to 'the past' as a concept in time.
A múlt mindig tanít valamire.
The past always teaches us something.
In this sentence, 'múlt' is used as an adjective modifying 'hét', referring to the previous week as a specific time frame.