Aldegunde,
"I work for only three hours a day",
and
"I work only three hours a day",
and
"I work for only a few hours a day",
and
"I work only a few hours a day",
are all perfectly acceptable.
If it makes you feel comfortable, teach your students to use "for" because this is correct. But where "for" is omitted, it is not necessarily incorrect.
The word "for" can be omitted sometimes, but I do not know the rule for when "for" can be omitted. If there is a rule, in English, I will try to find it for you
We always omit "for" before the word "all"
"Please, hurry! I don �t want to wait all day!" Not, " ... ... for all day!" The second sentence is not Standard English.
Do you read Japanese? You do! Then read this! I have improved this English summary for you.
This article aims to explore how and to what extent prepositions are left out in time adverbials in present-day spoken American English. On the whole, on, at, for, in, and during are dropped before time adverbials, and of those prepositions on and for are omitted most frequently in informal styles. "For" tends to be dropped when it is used with continuity verbs. In present-day informal American English, prepositions are omitted in time expressions to a considerable degree. In this article, the way and the extent to which prepositions are omitted in time adverbials are discussed based on a large number of examples.
Les
