Joe,  consider the following excerpt from a MODS record:

<titleInfo>

<title>Private Eyeballs</title>

<subTitle>a golden treasury of bad taste</subTitle>

</titleInfo>

…. and note that  <title> in MODS corresponds to BIBFRAME mainTitle so you could look at this as:

<titleInfo>

<mainTitle>Private Eyeballs</mainTitle>

<subTitle>a golden treasury of bad taste</subTitle>

</titleInfo>

(Even though there is no <mainTitle> element in MODS, pretend there is.)

So if you want to make this into a BIBFRAME title, the lazy way to do it might be to construct a title string from the main and sub title:

bf:title [rdfs:label “Private Eyeballs - a golden treasury of bad taste”]

Or, you could parse out the main and sub titles:

bf:title [

bf:mainTitle "Private Eyeballs" ;

bf:subtitle "golden treasury of bad taste" ] .

Or you could provide both the title string as well as the parsed title:

bf:title [

rdfs:label “Private Eyeballs - a golden treasury of bad taste” ;

bf:mainTitle "Private Eyeballs" ;

bf:subtitle "golden treasury of bad taste" ]  .

Let’s take another example.  In this MODS record, only the main title is expressed:

<titleInfo>

     <title>Reflexive interpreters and autobiographical texts</title>

</titleInfo>

So the mainTitle is “Reflexive interpreters and autobiographical texts”.  You could express this in BIBFRAME as the mainTitle, but only if you use that string verbatim. 

You could express the string:

     “Reflexive interpreters and autobiographical texts – English – 1977”

But in this case you would express it as a label, not as the mainTitle because it is not the main title.

Hope that helps.

Ray