Gentil vs Gentle

Gentil usually means nice or kind: Il a un gentil mot pour chacun - He has a kind word for everyone. It can also mean good, as in il a été gentil - he was a good boy.
Gentle can also mean kind, but in the more physical sense of soft or not rough. It can be translated by doux, aimable, modéré, or léger: He is gentle with his hands - Il a la main douce. A gentle breeze - une brise légère.

Grand vs Grand

Grand is a semi-false cognate. It means both great (e.g., un grand homme - a great man) and large or tall: elle est grande - she's tall, une grande quantité - a large quantity.
Grand is a very versatile term. It can mean large or impressive in size, scope, or extent; rich and sumptuous; pleasing; and/or important/principal.

Grappe vs Grape

Grappe is a cluster: une grappe de raisins - a bunch of grapes, grappes humaines - clusters of grapes.
Grape is un raisin.

Gratuité vs Gratuity 

Gratuité refers to anything that is given for free: la gratuité de l'éducation - free education.
Gratuity is un pourboire or une gratification.

Grief vs Grief

Un grief is a grievance: Il me fait grief d'être au chômage - He holds my unemployed status against me.
Grief refers to great sadness or chagrin: I'd never felt such grief - Je n'ai jamais senti une telle douleur.

Habit vs Habit

Habit means one's dress or outfit; Habits means clothes.
Habit refers to something a person does regularly, even to the point of addiction: habitude, coutume. It can also refer to a nun's outfit: habit de religieuse.

Haineux vs Heinous

Haineux is from haine - hatred. It means malevolent or full of hatred.
Heinous means atrocious/horrific: odieux, atroce, abominable.

Hasard vs Hazard 

Hasard is a semi-false cognate. In addition to hazard, it can mean coincidence, chance (e.g., a chance meeting), or fate. Par hasard - by chance.
Hazard refers to un risque, danger, or péril.

Ignorer vs Ignore

Ignorer is a semi-false cognate. It nearly means to be ignorant or unaware of something: j'ignore tout de cette affaire - I know nothing about this business.
Ignore means to deliberately not pay attention to someone or something. The usual translations are ne tenir aucun compte de, ne pas relever, and ne pas prêter attention à.

Inconsistant vs Inconsistent

Inconsistant indicates poor consistency: flimsy, weak, colorless, runny, or watery. In a more general sense, it can be translated by inconsistent.
Inconsistent means lacking consistency or being erratic: inconséquent, incompatible.

Instance vs Instance

Instance means authority, official proceedings, or insistence.
Instance refers to something that is representative of a group, an example - un exemple.

Intéressant vs Interesting

Intéressant is a semi-false cognate. In addition to interesting, it can mean attractive, worthwhile, or favorable (e.g., a price or offer).
Interesting means captivating, worth looking at, etc.

Intoxiqué vs Intoxicated

Intoxiqué means poisoned, while intoxicated means drunk - ivre.

Jars vs Jars

Jars is the French word for gander - a male goose.
Jars are wide-mouthed containers made of glass or pottery: pots, jarres.

Journée vs Journey 

Journée refers to a day.
Journey is un voyage or trajet.

Kidnapper vs Kidnapper

Kidnapper is the French verb to kidnap.
Kidnapper refers to the person who does the deed - un ravisseur / une ravisseuse.

Lac vs Lack

Lac is a large body of water - lake.
Lack is a deficiency or absence - un manque.

Laid vs Laid

Laid is an adjective meaning ugly.
Laid is the past tense of the English verb to lay: he laid his briefcase on the table - il a posé son porte-documents sur la table, she was laid on the ground - elle était déposée au sol.

Lame vs Lame 

Lame is a noun: strip (of wood or metal), slide (of a microscope), or blade.
Lame is an adjective: boiteux, estropié, faible.

Large vs Large

Large is the French adjective for wide, broad, or expansive. It can also mean generous or ample.
Large is synonomous with big - grand, vaste, gros, important.

Lecture vs Lecture

Lecture refers to reading in all senses of the word.
Lecture indicates a speech on a particular subject, especially for academic purposes: une conférence.

Librairie vs Library

Une Librairie is a bookstore, while Library in French is une bibliothèque.

Liqueur vs Liquor

Liqueur is a sweet, flavored alcoholic beverage: J'ai bu une liqueur après le dîner - I drank a cordial after dinner.
Liquor can mean any alcoholic beverage, but most often refers to hard liquor: Liquor is his only vice - L'alcool est son vice unique.

Lit vs Lit 

Lit is a bed.
Lit is the past participle of to light: allumer or éclairer.

Location vs Location

Location refers to something that is available for rent, such as a house or car. C'est pour un achat ou pour une location ? - Is it to buy or to rent?
Location indicates the position or placement of someone/something: It's a suitable location for a bakery - C'est une emplacement convenable à une boulangerie.

Logeur vs Lodger

Logeur is the landlord - the person who rents out rooms, while a Lodger is the opposite - the person who rents/stays in the rooms: locataire, pensionnaire.

Losange vs Lozenge 

Losange means diamond (in shape).
Lozenge is une pastille (pour la toux).

Magasin vs Magazine

Magasin is the general word for a store. It is also equivalent to the magazine of a gun. 
Magazine is une revue or un périodique.

Mail vs Mail 

Mail is the French word for the old-fashioned meaning of mall, i.e., a tree-lined walk or square.
Mail as a noun = poste or courrier; as a verb = envoyer or expédier (par la poste), poster.

Main vs Main

Main is the French noun for hand.
Main is the English adjective for principal, premier, majeur, or essentiel.

Malice vs Malice

Malice is a semi-false cognate; it can mean malice or simply mischievousnous or mischief.
Malice has only the stronger meaning of deliberate cruelty: méchanceté or malveillance.

Marron vs Maroon

While both of these are colors, Marron is brown and Maroon is a reddish color, best translated by bordeaux.

Mécanique vs Mechanic

Mécanique is an adjective which means mechanical or machine-made.
Mechanic is a worker skilled in making, repairing, or using machines: un mécanicien.

Menteur vs Mentor 

Menteur can be a noun - liar or an adjective - false. Mentir - to lie.
Mentor is a noun - mentor, maître spirituel.

Merci vs Mercy 

Merci i is the French word for thank you. It can also mean mercy, but this is quite uncommon.
Mercy refers to pitié, indulgence, or miséricorde.

Mère vs Mere 

Mère means mother.
Mere is an adjective meaning simple, pur, seul, etc.

Mode vs Mode

Mode is a semi-false cognate. Normally, it means fashion; à la mode literally means in fashion or fashioable.
Mode is a manner or way of doing something: a mode of life - une manière de vivre or a particular form, variety, or manner: a mode of communication - une façon de communiquer. It can also refer to status: The computer is in interactive mode - L'ordinateur est en mode conversationnel.

Monnaie vs Money

Monnaie can refer to currency, coin(age), or change.
Money s the general term for argent.

Mousse vs Mousse

Mousse is a semi-false cognate. It does refer to the dessert and hair product, but it also means such diverse things as moss, lather, or foam - so it's very important to pay attention to the context! This includes the foam in beer or a bottle of champagne.
Mousse simply refers to the dessert: chocolate mousse - mousse au chocolat or a styling product: hair mousse - mousse coiffante.

Mouton vs Mutton

Mouton can refer both to the animal (sheep) as well as the meat (mutton).
Mutton refers only to the meat.

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.