Putting it together
Mixed sentences drawing on Lessons 1–21, with nothing flagged in advance.
Nothing new here. These sentences are mixed on purpose: no lesson heading tells you which rule is being tested, which is exactly the situation you are in when you actually speak.
When a sentence stalls, work in this order: gender → case → ending. Almost every mistake at this level is a gender guessed wrong, not a rule forgotten.
Ich spreche mit dem neuen Kollegen.
I am talking to the new colleague.
dative from mit, -en on the adjective, -n on Kollege
Kannst du mir die Adresse schicken?
Can you send me the address?
dative pronoun, then the thing
Ich gebe ihm das neue Buch.
I am giving him the new book.
Leider kann ich heute nicht kommen, weil ich arbeiten muss.
Unfortunately I cannot come today because I have to work.
leider first, then verb-final after weil