Passive/active voice

[newyear2012]

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.

[billmcd]

Ex1:Understood. (not “understand” alone. Is it because of no subject and therefore switches to passive even if meant in present tense?) As in "It is understood" (subject "It") and portion of the passive ("is) is simply assumed. But you will hear, on occasion, "Understand" as a response to some comment.

Ex2:I understand. (seems 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.

b.

[newyear2012]

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?)