Htaccess and SSI questions

I've been digging around on Google for a few hours now and I'm not getting anywhere. All the 'guides' are basically copy and paste guides and don't give a lot of details, unfortunately.

I have read that Apache will not send the 'last modified' header for SSI pages, which may mean that browsers won't cache them.

According to various guides, I need to force the headers with a few different commands. The ones that seem relevant for me are:

ExpiresActive On
ExpiresDefault "access plus 1 month"

I've put this in an htaccess file in the appropriate sub directory that I want affected, along with the root htaccess info, i.e. hotlinking, ect.

I'm getting a 500 error with this new file. What am I doing wrong?

Also, I'd like to confirm a couple of things:

1) Htaccess works in top down fashion, i.e., an htaccess file in a sub directory will affect that directory and ALL of its' subdirectories, correct?

2) Subdirectory htaccess files completely replace ones that are higher up, iow, if I leave the subdirectory file blank, then that directory and all subdirectories would function as if there wasn't one, even if I have one in the root directory. Right?

3) The Expires directives affect subdirectories in addittion to the immediate directory?

I'd also appreciate any links to *really good* htaccess and SSI guides. Thanks.