Leono estas forta.
La dentoj de leono estas akraj.
Al leono ne donu la manon.
Mi vidas leonon.
Resti kun leono estas danĝere.
Kiu kuraĝas rajdi sur leono?
Mi parolas pri leono.
1. -J shows that there is more than one of the object to which it is attached (birdo = a bird, birdoj = birds). Adjectives accompanying a plural noun also have to have an attached -J (Akraj dentoj estas danĝeraj).
2. DE is probably the most heavily overused preposition in Esperanto. However, its most common use is to show possession. Here it is used in place of the English of, or the 's and s' on the end of a word.
3. -U on the end of the word shows a command, request or desire that the state or action described by the word be brought about, maintained or fulfilled. In other words, it's sort of like the English imperative or command mood, and when spoken to a second person, that's exactly what it is. Unlike the English, however, in Esperanto you can direct such a command, etc., to yourself or to some person who is not present (mi ne kuŝu sur la tablo).
4. -N is probably the most criticized ending in the language -- and certainly the most useful. When attached to a noun, it shows that the thing in question is either the object (recipient) of the action described in the verb, or (if the verb shows movement only) the destination of that movement. Its most common use is as the direct object of a verb -- that is, the thing at which the verb's action is directed. Important note: The existence of -N does away with the need for a strict word-order as a means of defining who does what to whom: La leono trovis birdon = The lion found a bird, but La leonon trovis birdo = A bird found the lion. Two other important notes: When we want to show a plural word as a direct object, the -N follows the -J (plumo -> plumojn); and, as with plurals, any accompanying adjectives must have the same object ending as the noun (La frato vidas la maturajn homojn).
5. -I marks a state or action in an undefined time -- what in English is called an infinitive. The Esperanto -I almost always corresponds to the English to with a verb (almost because in English, after a very few "helping" verbs such as can, the to disappears; but the -I on the equivalent Esperanto word never disappears).
6. KUN means "with", but only in the sence of in the company of. The English with has a number of unrelated meanings (she traveled with her sister, he fought with a sword, he fought with his enemies), the others of which are handled by other prepositions which we shall encounter later.
7. -E indicates that the word, like an adjective, is a descriptive word; but where an adjective describes only some thing (noun), these E-words (adverbs) describe states and actions (verbs), and also other descriptive words (adjectives and other adverbs). Notice above that when a sentence starts with a verb (in the infinitive), any associated descriptive word has to be an adverb rather than an adjective (again, adjectives go only with nouns).