Could you construct different sentences to stress "The dress is mine." vs. "My dress." or is there no difference? Is the meaning determined by context?
השמלה שלי ולא שלך, אל תלבש אותה - The dress is mine and not yours, do not wear it
אל תלבש את השמלה שלי - Do not wear my dress
So yes, it is determined by context. Actually the first one is a shortening of "השמלה היא שלי" (literally, "the dress is mine").
That should be the default shouldn't it? Wouldn't this sentence be השמלה היא שלי?
I would appreciate an explication of the rules that adjust the vowel sounds
dress is simlah but dresses is smalot, is it just memorisation or do certain verbs adjust vowels in predictable a way?
Well, there is a group of feminine nouns of the form XVXXa (X: consonant, V: vowel), which behave like segolates, for example: מַלְכָּה queen, גִּבְעָה hill, מִנְחָה tribute, grain offering, שִׁפְחָה female slave, handmaid have the plurals מְלָכוֹת, גְּבָעוֹת, מְנָחוֹת, שְׁפָחוֹת.