Since the verb is "ist" it doesn't take accusative form since it is the verb "to be". It is only accusative if it's an object (sentence wise). Also, accusative only affects masculine nouns. As and example, we could have: Ich mag den Hund, meaning I like the dog, but if we replace the verb with bin, it's ich bin der Hund. Hope this helps
Is that a kind of rule?
Short vowels are usually followed by a double consonant in writing, long vowels by just one.
Compare English, where "backing" and "baking" use different consonant spellings to mark the different vowels.
Neither language has phonetic double consonants (i.e. the double -tt- in "matting" is not pronounced any longer than the single -t- in "mating"); double consonants are simply an orthographic device to indicate a preceding short vowel. (Helpful, since both languages have more vowel sounds than vowel letters.)
It's not completely consistent, though; at the end of short words, the final consonant is often not doubled (e.g. hat in either language rather than hatt -- but it's doubled in er hatte or the mad hatter), so in single-syllable words in German, you cannot always correctly guess the vowel length. For example, Gas has a long a but was has a short one; er trat (he kicked/stepped) has a long vowel but er hat a short one.
See here: http://www.canoo.net/inflection/dein:Pron:Poss:2nd:SG In nominative, "deine" is used for feminine or plural nouns, "dein" for masculine or neuter nouns.
Could this be missheard as "das ist ein Schlüssel"?
Possibly, depending on how carefully the speaker was speaking.
Though if the speaker is speaking particularly "un-carefully" (conversationally), the distinction becomes clearer again: Das ist ein Schlüssel would sound like Das'n Schlüssel while Das ist dein Schlüssel would sound like Das' dein Schlüssel.
I understand the difference between dein and ein, but I can't tell them apart in the audio.