I moved (because of work) in a non-English speaking country, where pronunciation of English words / sounds can be a problem sometimes. I try to help them improve, since my level of English is much better (not native) than theirs.
One situation is regarding the words in the title.
The following sentences sound exactly the same when they speak:
I walked yesterday.
I worked yesterday.
So in these situations, I invariably have to ask for clarifications.
What I "trained" them to do is to keep the pronunciation for "walk" unchanged (similar to u-o-k), and to add a "faint" (20%..40%) "r" in "work" (similar to u-o-r-k).
Do you have a better idea? I usually have no problem in understanding the difference between the two when they are used by a native speaker.
Slightly different question for the same problem: is it very wrong to introduce that partial "r" (for the sake of disambiguation), even though in the text-book rule there is no "r"?