What happens if I use the wrong gender for a noun?
While native speakers will usually understand you, it sounds unnatural and can lead to errors in adjective and article declension. It's a common mistake but one to strive to correct.
Example
"Ich habe ein die Buch gelesen." (Incorrect) vs. "Ich habe ein Buch gelesen." (Correct, das Buch is neuter, ein is neuter accusative).