A key distinction in language learning is between ambiguity that exists at the word level versus information that simply isn't encoded in the source language at all. Many words appear ambiguous in isolation but become clear in actual sentences through grammar and context. For example, "lay" might seem confusing as an isolated word, but "I lay the book down" versus "I lay on the bed yesterday" are perfectly clear from sentence structure. Similarly, "certain" in isolation could mean multiple things, but "I am certain" versus "a certain person" are unambiguous in context. These cases require only reading comprehension skills - the information needed for translation is present in the sentence itself.
The genuine challenge arises when the source language fundamentally doesn't encode distinctions that the target language requires. English "my brother" provides no information about whether the sibling is older or younger, yet Chinese requires choosing between 哥哥 or 弟弟. English "he's gone" doesn't specify whether someone stepped out briefly or departed on a journey, yet Lithuanian distinguishes išėjęs from išvykęs. In these cases, the sentence itself - no matter how carefully analyzed - doesn't contain the information needed for translation. The learner needs either additional context beyond the sentence or real-world knowledge about the situation being described. This is the crucial teaching challenge: helping learners recognize when they must specify information that their native language leaves implicit.