what is the difference from the following sentences?

English as a Second Language English Vocabulary, Grammar and Idioms
1

i decided not to buy anything untill i had disembarked.
i decided not to buy anything untill i disembarked.

2

Strictly speaking, the first sentence more precisely says that you will buy something only after you have disembarked (rather than at the same time). However, in practice the two sentences would often be used interchangeably.

Note the spelling “until”, and note that the word “I” should always be capitalised.

3

You must disembarked first and then you could buy .

In the second sentence, before disembarking you decided not buy, but you may buy once you disembarked.