When Deeply Absorbed in Work, He Would Forget to Eat or Sleep: Is 'As' the Correct Word?

Yes, 'as' can be filled in the blank. The sentence means that when he was deeply involved in his work, he would often forget to eat or sleep. Here, 'as' can be understood as 'however' or 'no matter how'.

The sentence structure is a common pattern in English:

• Adverbial clause (When deeply absorbed in work) + Main clause (_______ he often was, he would forget all about eating or sleep.)

The adverbial clause modifies the main clause, indicating the condition under which the action of the main clause occurred. 'As' in this context acts as a conjunction, connecting the two clauses and highlighting the contrast between the deep absorption in work and the forgetfulness about basic needs.