Horn/Technique/Articulation and Tonguing
Articulation and tonguing are two closely related concepts, as much articulation is done with the tongue.
On most notes, the tongue is used to start the note. Release the air used to start the note with the syllable "tah". Make sure that the T is not too audible. Once the note has started, the tongue must be out of the way of the sound. Unless otherwise noted in the music, this is how all notes should be tongued. If a passage is to be played legato but not slurred, then the tongue should be light. In this case, it may be more helpful to think of "dah" rather than "tah".
When a slur is called for in the music, simply do not tongue any of the notes in the slurred passage. Slurs should sound beautiful and have a subtle transition from note to note. Slurs are indicated by a curve connecting the first note of the slur to the last note of the slur. It is generally acceptable to start a slur with the tongue.
Accents / Sforzandos
Accents should have more air behind them, but also are produced by tonguing harder. This will push more air at the beginning of the note. A sforzando is simply an exaggerated accent. An accent is indicated by a symbol looking a bit like > above or below the note. A sforzando is indicated by sf or sfz below the note. fp or sfp indicate that a note should be played sforzando then quickly reduced to piano (quiet).
Staccato notes may be slightly accented, and they should be short and separated. The shortness of the note is created by stopping the airstream. Do not stop the note with the tongue. Tongue-stopping will result in a choked sound.
Double and triple tonguing
Double and triple tonguing is useful in fast phrases where regular tonguing is not fast enough. For double tonguing, the first note should be tongued with the syllable "tah" like normal, but the second note is tongued with the syllable "kah". With practice, the tongue will be able to double tongue very quickly, allowing long sixteenth-note passages to be playable by simply silently saying tah-kah-tah-kah with the tongue. Triple tonguing is similar; it is produced with the syllables tah-kah-tah. This will come in very handy in passage in fast 6/8 time or passages with lots of triplets. Whenever double or triple tounging is used, there must be more air behind the "kah" or else it will result in an extremely weak articulation.