I'd use while travelling.Originally Posted by alan
when he travelled last year functions as an adverbial clause. when is not a verb, so it doesn't have a verbal form. It joins the clause He bought something with the clause he travelled last year, making it a conjunction. If it were something else, say a relative adverb, it would modify the word 'place', like this,Originally Posted by alan
He bought something at that place where they have fine wines.
Well, here's a trick to find out: expand the sentence, like this,Originally Posted by alan
While he was travelling, he bought something.
travelling functions as a participle in that context.
They are the same. 'we are' is omitted because 'we' is redundant. It's stated in the main clause. 'are' is omitted because it goes hand-in-hand with 'we', the subject. If we omit 'we', we have to omit its verb, 'are'.Originally Posted by alan