Napkin vs Napkin

Napkin is not in any of my dictionaries, but I learned the hard way :-) that it means a sanitary napkin: J'ai besoin d'un napkin - I need a sanitary napkin.
Napkin is correctly translated by serviette: I need a napkin - J'ai besoin d'une serviette.

Nature vs Nature

Nature can be the noun nature, but is also an adjective that means plain, natural, or uninhibited.
Nature = la nature.

Niche vs Niche

Niche is a semi-false cognate. In addition to niche or recess, it can refer to a kennel or doghouse.
Niche means une niche, un créneau, or une voie (when used figuratively: His niche in life - Sa voie dans la vie).

Note vs Note

Note is a semi-false cognate. In addition to note, it can refer to scholarly marks or grades or to a bill, check, or account.
Note is translated by une note except in the sense of a short letter - un mot.

Notion vs Notion

Notion refers to a conscious notion, as in "Je n'ai aucune notion de cela" - "I have no notion about that." It can also mean elementary knowledge, such as "J'ai quelques notions d'arabe" - "I know a smattering of Arabic."
Notion usually means une idée.