Grammatical agreement is a vast topic - and one of the banes of French students. While in English we have a few pronouns and adjectives that indicate gender and number (e.g., he/him/his and she/her/hers), in French, agreement is found in 5 of the 8 parts of speech.
An article is a word that modifies a noun in a particular way, by stating whether the noun is specific, unspecific, or partial. There are three types of French articles, and they all agree in gender and number with the nouns they modify.
The preposition de can be very difficult for French students, even at advanced levels. Knowing whether to use du, de la, or des rather than just de can be a real challenge! This lesson is a detailed explanation of when to use the preposition de all by itself and when to use the indefinite article, partitive article, or de + definite article (which looks like the partitive - but isn't. Ugh!)
The French definite article (le, la, l', les) indicates either a particular noun or, contrarily, the general sense of a noun. It's used similarly to its English counterpart, but there are many instances where a definite article is required in French but not English.
With articles, the de vs du, de la, des choice has to do with affirmative/negative and whether there's an adjective in front of the noun.
The aptly named indefinite article (un, une, des) indicates an unspecific or unidentified countable noun.
What's the difference between on and l'on? In a nutshell, on is sometimes preceded by l' for reasons of euphony.
The partitive article (du, de la, de l', des)refers to an unspecified quantity of food, liquid, or some other uncountable noun. English has no equivalent article - the partitive is usually translated by the adjectives "some" or "any," or may be left out entirely.
Generally speaking, articles are much more common in French than in English, but there are exceptions, such as when certain prepositions are followed by nouns.
Do you know the difference between un and l'un? If you answered, "Huh? Why would you ever put l' in front of un?" then this is the lesson for you.