Ex1:Understood. (not “understand” alone. Is it because of no subject and therefore switches to passive even if meant in present tense?)
Ex2:I understand. (seem this is the another alternative but with subject taking the action thus “understand” is in active.)
The context is response situation at least.
Seeking explanation/confirmation for example 1.
ex1: understood.( since the "it is" part can be implied in this structure without actually stating so, can "i" also work too in similar fashion?)
ex2: used to be very strong.(can "i" be implied in the same fashion as well?)