There is a small group of nouns that add an additional n to their ending IF they are NOT the subject of the sentence. The good news is that this concerns only masculine words [=der] mostly with very specific endings and as the only exception the word das Herz.
Whenever you find the endings -e, -ent, -ant, -oge, -ist, -graf, -at and the following words [this list is incomplete but sufficient]: Architekt, Bär, Herz, Herr, Mensch, Nachbar add an (e)n. A few fresh examples:
Ein Mensch liebt einen Bären.
A human being loves a bear.
Die Frau liebt einen Psychologen.
The woman loves a psychologist.
Das Auto des Fotografen ist kaputt.
The car of the photographer is broken.
Many masculine words ending in -e are found in nationalities: der Pole, der Russe, der Chinese, der Deutsche, der Vietnamese, der Franzose
THE PLURAL of all of these words is always identical to the n-version.
How to memorize them
You can learn the endings and the six words above with the help of a little story. Imagine the following scenery as vividly as possible and repeat it a few times over the next couple of days to imprint the information securely in your memory:
Mnemo-story to memorise N-declination signals
Read the story below slowly. After every sentence, visualize in your mind what you have read.
The President of your country is visiting India.
The President of India invites him to ride an elephant.
It's a huge elephant, so huge that quite a few people fit into the basket that the elephant carries on its back. The President never travels alone.
He always takes a diplomat with him.
And of course the press is also there: one chosen journalist.
As your president is keen on learning something new, he hired a pedagog.e to teach him about India's customs.
Suddenly the elephant comes to a halt. A bear (=Bär) stands in front of them causing
everyone to freeze.
But then along comes a Sir (=Herr) who tames the bear and claims that he is
its owner.
And he says that the bear has a good heart Herz.
It even has a tattoo shaped like a heart on its chest.
The Sir is in fact, a neighbour (=Nachbar) of the Indian President. Then, suddenly, a miracle occurs: there's a flash and the bear turns into a human being (=Mensch).
Who would have thought of that?
This topic is covered in: B2-L16