What is the difference between もう少しで and ほとんど?

Question

Consider these sentences:

1. (×ほとんど/○もう少しで) いうのを忘れるところでした。
2. プールの中の水は、(○ほとんど/○もう少しで) 満杯である。
3. 宿題は、(○ほとんど/×もう少しで) 終わった。
4. (×ほとんど/○もう少しで) あなたのお家に着きます。

In some cases it is ok to use もう少しで but in others it is not. The same goes for ほとんど. Sometimes both can be used.

I have heard explanations like ほとんど is mainly used for "possibilities" and are used for 1-D things like time, distance or level and things that have a "span". And もう少しで is used for possibilities. But I have no idea what this means at all.

Can someone explain the grammar behind this and why sometimes they are interchangeable and sometimes they are not?

Answer 1

English "almost X" can describe two different kinds of situations:

1. mostly X but not completely X

   • It's almost perfect (but is still imperfect).
   • I almost forgot it (my memory about it is already vague).

2. X was likely to happen, but it never happened

   • I almost died (but I didn't die).
   • I almost forgot it (but I perfectly recalled it before a problem arose).

Now...

• ほとんど is used only in Situation 1.

  • ほとんど完璧です。 It's almost perfect.
  • 彼はほとんど死んでいるようなものだ。 He's half dead.
  • 三角関数はほとんど忘れました。 I almost forgot about trigonometric functions.

• もう少しで is typically used in Situation 2. The pattern to memorize is もう少しで + ～するところだ, もう少しで + ～しそうになる. You can also use 危うく. See your own previous question and Learn JLPT N2 Grammar: ところだった (tokoro datta)

  • もう少しで死ぬところだった。 I almost died.
  • おっと、もう少しで忘れるところでした。 Oops, I almost forgot.

• もう少しで can be (usually) used in Situation 1, too. Still, もう少しでX basically means X has not happened yet or X did not happen. Try thinking this phrase as "with a little more time/effort/resource/luck/misfortune/etc, ..." and see if it fits the context.

  • もう少しで満杯です。 With a little more water, it will be full.
  • もう少しで終わります。 With a little more time, it will be done.
  • もう少しで勝てた。 With a little more luck/effort, we could win. (i.e., we lost)
  • 彼はもう少しで死ぬ。 He will die soon / with a little more damage.
  • ×もう少しで終わった。 (weird)

Typically, もう少しで is closer to "soon" rather than "almost". Compare:

• もう少しで春です。 Spring will come soon (but it has not come yet).
• もうほとんど春です。 It's already almost spring (it's warm).

Answer 2

もう少しで implies that the action or result of a certain verb is not completed/finished but its completion comes soon, so it goes with the present tense or the future tense like もう少しで終わる or もう少しで終わるでしょう/終わるだろう but doesn't go with the past tense like もう少しで終わった.
If you want to say something in the past tense using もう少しで, you can make it by constructing a sentence in the past tense which encompasses the present tense like もう少しで終わるところだった, which means 実際{じっさい}にはその時までには終わらなかった Actually it was not completed/finished before the designated time.

On the other hand ほとんど implies that the action or result of a certain verb is mostly completed/finished, so it goes with the past tense or verbs that express the state of things like (宿題｛しゅくだい｝は)ほとんど終わった, (宿題は)ほとんど終わる or (宿題は)ほとんど終わるでしょう/終わるだろう.