When is <verb stem>+かける's meaning to start and leave unfinished?

Question

I recently learned that the construction +かける means to start something, but with an implication that the action was left unfinished:

ビールを飲みかけたことがあるんだけど、味を全然好きなかった。

"I've had a beer before, but I didn't like its taste at all." (かけた here tells us I did not finish the beer.)

But I've also learned that in certain cases, there is no such implication of something being left unfinished:

だんだん、彼の気持ちがわかりかけてきた。

"I have gradually started to understand his feelings."

(This example is from maggiesensei.com)

How can I tell when a sense of incompletion is implied? Not knowing this could lead to some confusion and poor translation.

Answer 1

Unfortunately your first example doesn't tell the same meaning as your English. You have to say:

ビールを飲んでみたことがあるんだけど、味が全然好きじゃなかった。

Plainly speaking, ～しかける has only one meaning: aborting before the action reaches the "effective" stage. What means by "effective" is different according to verb (see Aktionsart), and often to each situation i.e. the moment you concern most in the action when you use the verb. For example:

グラスの水を飲みかけて、酒だと気づいた。
I nearly drank water in the glass when I realized it was sake. (not mouthed yet)

餅を丸ごと飲みかけて、死ぬかと思った。
I all but swallowed a whole piece of rice cake; I thought I was going to die. (already mouthed)

令和になったのに書類に平成と書きかけた。
I almost wrote "Heisei" into the form though it's already Reiwa. (not put pen yet)

半分まで書きかけた本を完成させたい。
I want to finish my halfway written book. (already written a lot)

From the second example in your question we can read that the speaker wants to fully understand his feeling, and until that the "understanding" is "incomplete". Thus the usage is valid.

On the other hand, people usually drink beverage to taste it, so it's normally understood that 飲む is "complete" when you take in beer. It's invalid unless you have some complicated context e.g. the speaker wants to poison the target etc.