The word IN is a preposition and it can be followed by either the ACCUSATIVE case or the DATIVE case, depending upon its meaning in the sentence. If the preposition is used with a verb of movement ( arbeitenfliegengehenkommenlesenspielen (to go), fahren (to travel),  arbeitenfliegengehenkommenlesenspielen (to come),  arbeitenfliegengehenkommenlesenspielen (to fly) ...) and the sense is INTO, then IN is followed by words in the ACCUSATIVE CASE. IN is thus showing MOVEMENT. If the preposition is used with verbs which show something being done in a certain place ( arbeitenfliegengehenkommenlesenspielen (to play),  arbeitenfliegengehenkommenlesenspielen (to work),  arbeitenfliegengehenkommenlesenspielen (to read), sein (to be) ...), then IN is followed by words in the DATIVE CASE. IN thus showing POSITION. Here are the various words for THE in the ACCUSATIVE and DATIVE cases, with the basic NOMINATIVE too:

	Masculine	Feminine	Neuter	Plural
NOMINATIVE:	der	die	das	die
ACCUSATIVE:	den	die	das	die
DATIVE:	dem	der	dem	den

NOTE: In the ACCUSATIVE case, IN + DAS usually contracts to become INS. In the DATIVE case in + DEM usually contracts to be come IM. Examples: Accusative: Ich gehe in die Küche (moving through the door into the kitchen = MOVEMENT. Dative: Ich koche in der Küche. (doing the activity in the kitchen = POISITION. Accustaive: Mein Bruder geht ins Kino. (moving into the cinema, so MOVEMENT. Dative: Mein Bruder sieht einen Film im Kino. (doing the activity in the cinema, so POSITION.

Here are the various words for A in the ACCUSATIVE and DATIVE cases, with the basic NOMINATIVE too:

	Masculine	Feminine	Neuter
NOMINATIVE:	ein	eine	<ein
ACCUSATIVE:	einen	eine	ein
DATIVE:	einem	einer	einem

And finally the various words for MY in the ACCUSATIVE and DATIVE cases, with the basic NOMINATIVE too.
Other possessive adjectives (dein, sein, ihr, unser, eur, ihr, Ihr) also have the same endings:

	Masculine	Feminine	Neuter	Plural
NOMINATIVE:	mein	meine	mein	meine
ACCUSATIVE:	meinen	meine	mein	meine
DATIVE:	meinem	meiner	meinem	meinen

The word IN is a preposition and it can be followed by either the ACCUSATIVE case or the DATIVE case, depending upon its meaning in the sentence. If the preposition is used with a verb of movement ( (to go), fahren (to travel), (to come), (to fly) ...) and the sense is INTO, then IN is followed by words in the ACCUSATIVE CASE. IN is thus showing MOVEMENT. If the preposition is used with verbs which show something being done in a certain place ( (to play), (to work), (to read), sein (to be) ...), then IN is followed by words in the DATIVE CASE. IN thus showing POSITION. Here are the various words for THE in the ACCUSATIVE and DATIVE cases, with the basic NOMINATIVE too:

	Masculine	Feminine	Neuter	Plural
NOMINATIVE:	der	die	das	die
ACCUSATIVE:	den	die	das	die
DATIVE:	dem	der	dem	den

NOTE: In the ACCUSATIVE case, IN + DAS usually contracts to become INS. In the DATIVE case in + DEM usually contracts to be come IM. Examples: Accusative: Ich gehe in die Küche (moving through the door into the kitchen = MOVEMENT. Dative: Ich koche in der Küche. (doing the activity in the kitchen = POISITION. Accusative: Mein Bruder geht ins Kino. (moving into the cinema, so MOVEMENT. Dative: Mein Bruder sieht einen Film im Kino. (doing the activity in the cinema, so POSITION.

Here are the various words for A in the ACCUSATIVE and DATIVE cases, with the basic NOMINATIVE too:

	Masculine	Feminine	Neuter
NOMINATIVE:	ein	eine	<ein
ACCUSATIVE:	einen	eine	ein
DATIVE:	einem	einer	einem

And finally the various words for MY in the ACCUSATIVE and DATIVE cases, with the basic NOMINATIVE too.
Other possessive adjectives (dein, sein, ihr, unser, eur, ihr, Ihr) also have the same endings:

	Masculine	Feminine	Neuter	Plural
NOMINATIVE:	mein	meine	mein	meine
ACCUSATIVE:	meinen	meine	mein	meine
DATIVE:	meinem	meiner	meinem	meinen