ーたら、....

The combination of V(Past Plain Form) (or -た form)+ら carries the meaning of 'when/if'.

There is a strict restriction on the usage of V-たら. That is, in [V1-たら、V2。]
the action denoted by V1 must precede the one denoted by V2.

Consider the following examples.


The combination of -たら+どうですか, on the other hand, expresses one's suggestion/advice, and it corresponds to English "How about doing such and such?"/"Why don't you do such and such?"