It can often seem that while there are many grammar rules, there are just as many exceptions to them.
The rules and exceptions often have no rhyme or reason, and are simply structures to be learned until it becomes automatic.
Here are three common exceptions to look out for:
Colours may upon first glance seem like a simple process, when in actual fact they follow many rules.
Generally speaking, colour adjectives agree with the noun they follow, for example; les chaussures jaunes.
There are exceptions. If a colour gets its name from a flower, fruit or animal, this rule does not apply and the colour does not agree with the noun.
Examples are orange, marron, cerise, crème, pastel, turquoise, and bronze. These colours will keep their standard form and not change to agree with the noun.
With the colours rose, mauve and pourpre which all come from flowers or animal names, they do take the s in the plural; for example des pantalons roses.
Meanwhile, colour adjectives modified by another adjective or noun also vary - les yeux vert clair (clear green eyes - in this case the vert is modified by the clair so will not agree with the plural form).
Finally, when there are two colours together make sure to include a hyphen, for example une facade bleu-gris.
Plural compound noun agreements
Agreements are often something that confuse people and that is usually because of the rule exceptions.
Compound nouns are nouns made up of two or more words.
Normally words that form a plural compound noun add ‘s’ - eg. Des belles-sœurs (sisters-in-law).
However, here comes the exception to the rule.
When these compound nouns are formed of words that further define the noun, only the noun itself is written in the plural.
For example, une arrière-cour (backyard) becomes des arrière-cours (backyards) because arrière is further defining the noun cour.
Another exception is when the compound is made up of a verb and noun.
In this case if the verb is concrete and countable, the noun becomes plural and the verb remains the same - for example un couvre-lit becomes deux couvre-lits.
Meanwhile if the noun is abstract or there is only one of it, it is not written in the plural. For example, un gratte-ciel (a skyscraper) is des gratte-ciel (skyscrapers) in the plural.
This is because there is only one sky.
Saying this, a spelling rule introduced by a 1990 reform does allow for all nouns to be written with an s. However this is still not really common practice so you are most likely to see the exceptions to the rules still in play.
Adverbs ending in -ment
The basic rule here is that usually, there are three ways to form adverbs.
Adverbs ending in -ment can be formed by:
taking the adjective in its feminine form as the base and adding -ment: eg. généralement (général = adjective, générale = feminine adjective, + ment = généralement); and sûrement (sûr = adjective, sûre = feminine adjective, + ment = sûrement).
the adjective in its masculine form + ment when it ends in i, u or é: examples are vraiment (vrai = masculine adjective + ment) and absolument (absolu = masculine adjective + ment).
Meanwhile, adverbs end in -emment and -amment when the adjective ends in ent or ant.
For example, constant becomes constamment and apparent becomes apparemment.
However, as always there are exceptions to this rule.
The most common ones to look out for are gentil becomes gentiment as an adverb, while bref becomes brièvement.