mitteilen
to inform
To give someone information or make something known; to inform someone about a matter.
Der Arzt teilte dem Patienten die Ergebnisse der Untersuchung mit.
The doctor informed the patient of the examination results.

to communicate
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to inform
To give someone information or make something known; to inform someone about a matter.
Der Arzt teilte dem Patienten die Ergebnisse der Untersuchung mit.
The doctor informed the patient of the examination results.

The verb is made up of 'mit' (with) and 'teilen' (to share/divide). The literal meaning is therefore to 'share' information 'with' someone.
Können Sie mir bitte Ihre neue Adresse mitteilen?
Could you please inform me of your new address?
'Mitteilen' (to inform/communicate) is a separable verb. In a main clause, the prefix 'mit' goes at the end; in a subordinate clause, the verb stays together.
Der Chef teilt uns die Neuigkeiten morgen mit.
The boss will inform us of the news tomorrow.
The person who receives the information is always in the dative. The information itself is in the accusative. The pattern is: jemandem (dative) etwas (accusative) mitteilen.
Ich habe ihm meine Entscheidung bereits mitgeteilt.
I have already informed him of my decision.
'Mitteilen' (to inform) is more formal than 'sagen' (to say) or 'Bescheid geben' (to let someone know). It is frequently used in written language, in emails, or in official contexts.
Wir teilen Ihnen mit, dass Ihr Antrag genehmigt wurde.
We hereby inform you that your application has been approved.
In the reflexive form 'sich mitteilen' (to express oneself), the focus is on expressing one's feelings and thoughts. It is often combined with adverbs such as 'kaum' (hardly), 'schwer' (with difficulty), or 'offen' (openly).
In der Therapie lernte er, sich besser mitzuteilen.
In therapy, he learned to express himself better.
to say
'sagen' and 'mitteilen' both have to do with someone giving information. 'sagen' is very general and fits in almost any everyday situation, including short sentences or personal conversations. 'mitteilen' sounds more precise and a little more official: information is deliberately passed on to someone. In letters, emails, or important messages, 'mitteilen' is often more natural. In casual conversation, people usually just say 'sagen'.

Die Schule teilt den Eltern mit, dass der Unterricht morgen später beginnt.
The school informs the parents that classes will start later tomorrow.
Julia sagt ihrer Mutter vor dem Termin schnell die neue Uhrzeit.
Before the appointment, Julia quickly tells her mother the new time.