The way you phrased your question doesn’t seem quite right to me. An aspect marker may or may not be added to a verb. When it is, the resulting inflection still contains a part that indicates tense at the end. The question of which of る and た is more appropriate is a question about tense. What the book explains about 相対テンス and 絶対テンス in a noun-modifying clause still applies, with or without an aspect marker.
The first sentence would be understood as meaning either of the following:
- the speaker is currently studying with the man [continuous action]
- it’s the speaker’s current habit to study with the man [habit]
Either way, this is not what you want. If the man and the speaker were studying together at some point of time in the past, the verb form has to be 勉強していた. According to 視点の原理 in the book, this is an example of 絶対テンス as the same tense marker, た, is used in both the subordinate clause and the main clause.
First of all, 弟が sounds more natural than 弟は as a standalone sentence. The latter makes the sentence sound as if it’s talking about what the brother did on the assumption that he having done something is already known. If the sentence is about what happened with no such assumption, it should be 弟が.
Both are possible, but they conjure up slightly different pictures. In the first I see the brother eating from the speaker’s bowl while the latter is still in the middle of eating it, perhaps taking advantage of a moment when the speaker is looking the other way. In the second, on the other hand, I see the brother eating the noodles while the speaker is away, leaving the bowl unattended. The first sentence could be understood as describing the latter scenario as well, though, if the speaker is thought to have been still in the middle of eating the noodles.
The first sentence is definitely a case of 相対テンス as the subordinate clause and the main clause use different tense markers, one る and the other た. As for the second, while it might look like 相対テンス, I think it is more natural to understand this 食べていた as describing a past state in 絶対テンス.
The first sentence would mean that the brother found the toy he would eventually lose. So, the correct option has to be the second. However, the sentence would sound much more natural if it said 無くしてしまっていた玩具, or more simply 無くしていた玩具, to describe it as a past state (which resulted from a past-of-past action). I see this as a case of 絶対テンス, too. The toy was in the state of being lost at the time, as seen from the perspective of the speaker at the present.
In any case, 無くしてしまった itself doesn’t describe a past-of-past action as you seem to think it does. If it’s understood as one, that’s because of context. You must have lost the toy first to rediscover it later. This is probably the reason the sentence doesn’t quite sound natural.
a. 主節と従属節 (名詞修飾節を含む) が同じ時制形式 (ル形－ル形、タ形－タ形) である時、従属節のテンスは発話時との時間的前後関係で決まる (=従属節のテンスも絶対テンスになる)。
b. 主節と従属節 (名詞修飾節を含む)が異なる時制形式 (ル形－タ形、タ形－ル形) である時、従属節のテンスは主節時との時間的前後関係で決まる (=従属節のテンスは相対テンスである)。