Letters A, O, E, I, U are vowels. Their sound is self-standing.

All other letters are consonants. Their sound is not easily uttered in isolation: to spell "l" an "e" (a vowel) must be added to it: "el".

In English "y" performs as a vowel in some words (early, system), as a consonant in others (you, yes).

Phonetics differentiates between long and short vowels. Let us consider vowel "a" for an example: its sound is long in "father" and is short in "tackle".

Consonants may be written simple or double - with the difference resulting in a reinforced sound in some languages (not in English, which sounds double consonants like single ones).

As to diphthongs, some have a vocalic sound (island, ousting) and some a consonant sound (university, Europe). This matters in selecting the appropriate English indeterminate article ("a" before a consonant, "an" before a vowel):

• a European car
• an ousting of personnel

Diphthong:

(From Greek di- (two) and phthongos (sound).)

Four definitions follow:

1. A speech glide that begins with one vowel and gradually changes to another vowel within the same syllable, as (oi) in "boil" ["boil"] or (ai) in "fine" ["fine"]. The American Heritage® Dictionary of the English Language
2. A vowel sound in which the first vowel is gradually raised by a second vowel so that both vowels form one syllable (as “a” + “i” in "rail"). MSN Encarta
3. A sound produced by moving from one vowel sound to another within a single syllable. The Wordsmyth English Dictionary
4. A sound formed by the combination of two vowels in a single syllable (as in "coin") Compact Oxford English Dictionary

To gain an idea of what this is about, an excerpt drawn from a course of German follows. (For your information: "accusative" is the case into which "objects" are set in case-dependent languages.)

Accusative is mandatory after some verbs (in the so called direct construction, i.e. a noun follows a verb with no connecting prepositions in between).

Most verbs requiring direct construction in German do so in English too, but not all: "sprechen" (to speak, marked with asterisk) is directly constructed in German only:

• anrufen = to call somebody by phone
• fragen = to ask somebody
• sprechen* = to speak (to) somebody
• brauchen = to need somebody / something
• kosten = to cost somebody something

Wen rufst du an? Ich rufe den Schneider an.
    Whom are you calling? I call the tailor.
Ich frage deinen Bruder: "Kommst du mit?"
    I ask your brother: "Are you coming along?"

[ mitkommen = to come (kommen) with (mit) ]
Wir sprechen den Chef.
    We speak to the boss.
Ich brauche einen Bleistift
    I need a pencil.
Das kostet mich viel Mühe.
    That costs me a big effort.
[die Mühe = fatigue, weariness]
French and other languages do not have the "double accusative construction" found in English and German: compare "I ask your brother something" with "Je dis quelque chose à ton frère". ("À ton frère": an indirect construction.)

With subject-predicate inversion*, personal pronouns may end up before the subject:

Gestern hat ihn der Leiter angerufen.
    Yesterday the (school) principal phoned him.

* The swapping of subject and predicate is called inversion.

[For more on subject/predicate inversion, see article.]

Inversion exists in English too, but it is less frequent than in German:
I watch movies only rarely.
Only rarely do I watch movies.

Whilst in English we insert "to do", in German subject and predicate just swap places.

Within a non-negative main clause, the verb takes second place. German leaves you free to begin a clause with subject or complement. But - when the subject is not at the beginning - then it is moved after the verb:

Es hat gestern stark geregnet »
Gestern hat es stark geregnet.
    Yesterday it has rained heavily.

If the clause begins with "ja", "nein", "doch", "aber", "sondern", "denn", "oder", "und" and exclamative marks ("Mein Gott" [my God], "Mensch!" [oh, man!]) verb and subject don't swap places:

Ja, ich komme sofort.
    Yes, I come right away.
Nein, ich habe ihn nicht angerufen.
    No, I haven't phoned him.
Er ließ das Fenster zu, aber er machte die Tür auf.
    He kept the window shut, but he opened the door.
Er half der Frau nicht beim Aussteigen, denn er hatte es eilig.
    He did not help the lady to step down, because he was in a hurry. [... had it hurriedly]
Er schneidet den Artikel nicht aus der Zeitung aus, sondern er schreibt ihn ab.
    He doesn't clip the article off the newspaper, but he copies it.
Der Lehrer schreibt den Satz an die Tafel, oder (er) diktiert ihn.
    The teacher writes the sentence on the board, or he dictates it.
Mensch, ich halte dich nicht mehr aus!
    Gosh, I don't stand you anymore!