Yeah, you can copy characters, backgrounds and assets from one FLA to another. Also, get into the habit of saving versions of your file as you progress - like FileName_01.fla, FileneName_02.fla and so on, to have backups and prevent issues with work loss due to data corruption. What do you mean by 'marking' an object? Do you want to select a raw shape and its outline? You can click the stroke and then hold down Shift and click the fill; or you can do a box selection (click outside and drag), or click on the keyframe in the timeline, or select with the lasso tool, and then either convert to a symbol or a Union/Drawing Object. If you are new to Animate, you'd better get to know some basics:
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