ないことはない: admitting a possibility with reservations
Learn a qualified affirmation formed through double negation. Distinguish potential and ordinary negative forms without expanding the meaning into lack of necessity or enthusiastic agreement.
Not going so far as to call it impossible
一人で運べないことはありません。 means that carrying it alone is not impossible. Negating the negative potential 運べない admits feasibility, but differs from confidently saying the task is easy.
時間をかければ、今日中にできないことはありません。 says that it can be done today if enough time is devoted to it. Do not remove the condition and interpret completion as unconditionally easy.
A qualified affirmation after an ordinary negative
食べないことはありませんが、あまり好きではありません。 means that the speaker does eat it, but does not particularly like it. 食べない is an ordinary negative, unlike 食べられない focusing on ability to eat.
A following が can add a burden or reservation. Even without an explicit reservation, do not turn the qualified affirmation into strong enthusiasm or wholehearted agreement.
Different from not having to act
行かなくてもいいです。 permits not going or says it is unnecessary. 行けないことはないです。 admits that going is possible, so separate necessity from feasibility.
Start by identifying the verb form. 分からないことはない says that understanding is not entirely absent; it does not guarantee perfect understanding of every detail.
Quick check: Does 高いですが、買えないことはありません。 conclude that buying it is impossible?
Show the answer and why
No. It says the item is expensive, but buying it is possible. Feasibility is admitted alongside the burden of the price.