In another thread, they said that יכול implies more of a physical possibility than English, and therefore it doesn't apply here except that you try to say that they are actually unable to find us (like maybe we are dead or in the other side of the galaxy so there's no chance of them finding us).
The groupings of verbs. I think there are seven. It's in the tips and notes. You cannot see it in the app. There is a full breakdown on Pealim.com of the different groups. I think it's like one is for action verbs, passive verbs, etc. But I'm learning too...
Couple ways if you're in the app to get the tips and notes for each skill (off Duolingo's website):
can also get the tips and notes, organized by skill: http://duolingo.wikia.com/wiki/Hebrew
Or organized by skill in one pdf for the whole course: https://www.docdroid.net/JnfmyEV/tipsnotesbackup.pdf. (I downloaded this, it's the same guy who made the Memrise Duolingo Hebrew vocabulary course, it's safe & legit.
Replace your username where mine is for information on your progress, etc: https://duome.eu/teribleteri/progress for progress AND for the tips/notes by skill.
From the duo tips and notes, taken from duome.eu:
Binyanim ("constructions") are a formulaic way of creating lots of different verbs in Hebrew in a predictable way. There are seven binyanim and each binyan is a set of patterns for each tense.
Most verbs in Hebrew have a 3 letter root (some have 4) from which you can derive every kind of word and it is this root which is inserted into a binyan - a set of patterns that can be a mixture of prefix letters, suffix letters and vowels.
This is not such an alien concept for speakers of English after all. For example, the sounds "s" and "ng" produce words with several different but related meanings when different vowels are inserted in between: "sing", "sang", "sung", "song".
Some roots are expressed in several of the binyanim, while others only exist in one binyan (don't be intimated by the system of binyanim - you don't have to learn several conjugations for each root word). To put it basically, three of the binyanim are for active verbs ("doers" of the action) and three are for passive verbs (receivers of the action), while the final one is usually for reflexive actions (an action done to oneself). Other differences between the binyanim can be to do with whether the verb is transitive or intransitive ("the boy grows" and "the boy grows plants" use different binyanim), or causative ("write" versus "dictate", "learn" versus "teach"), but we will come to all this in due course...
(It goes on for a page or so... This is only the intro to this skill's notes).