Translation:He was learning to do it with a new tool.
They are not interchangeable and they were not interchangeable in previous lessons. "A" and "the" are used in different situations in English which may or may not correspond to different sentences in Czech.
In certain sentences using "the" if there is no demonstrative pronoun in Czech, is accepted if such a sentence makes good sense - when it would be understood from context which particular item we are speaking about. I do not see that likely to be said this way here.
Vladafu is usually so helpful but his comment on "the" seems inappropriate here (the "the" does contribute to a sensible sentence). The wording is slso somewhat unclear. I could use some help with this imperfective, though. What about "učil se" marks it as imperfective? Thank you! Sorry if this is explained on the PC version. I just have my phone.
Here's a brief explanation of imperfective/perfective verbs that you might find helpful as a starting point: https://mluvtecesky.net/en/grammar/verbs. (It's pretty near the bottom of the page.)...
----- "Most Czech verbs "live in a pair", it means they have the imperfective and perfective aspect, for example dělat/udělat. The aspect is a verbal category by means of which a verb expresses a process or activity from the point of view of incompletion or completion, repetitiveness or one-time-activity.
Imperfective verbs express an action as a process. These verbs express unlimited, unfinished or repeated action. They have past, present and future tense forms.
Perfective verbs express a result of an action or an action in its specific moment, for example at its beginning or end. Perfective verbs do not make present tense forms, they have only past and future tense forms. Note: The forms of perfective verbs with present tense forms actually express a future meaning, for example: Zaplatím. Sejdeme se v 6 hodin. Zavolám ti/vám.
Some verbs have no perfective counterpart, for example modal verbs (chtít, moct, muset) or state of being verbs (být, spát, stát)." -----
With regard to učit se specifically, dictionary entries will identify it as imperfective (nedokonavé), as, for example, here: https://cs.wiktionary.org/wiki/u%C4%8Dit_se.