In Arabic, there is one definite article, called the alif-laam, spelled "ال" or 'al-,' which connects to the beginning of the word. This never changes due to number, gender, or case, as it does in many European languages. However, there is one rule, which has nothing to do with grammar, but everything to do with spelling and pronunciation.
Every Arabic letter falls into one of two categories: "الحروف الشمسية" 'al-Huruuf ash-shamsiyya'--'Sun letters,' and "الحروف القمرة" 'al-Huruuf al-qamariyya.'--'Moon Letters.' In the first category, the Sun Letters, the laam of the alif-laam is not pronounced like /l/, but is 'swallowed' by the following consonant, which is then pronounced with a 'shadda,' that is, it is doubled. The alif-laam in the second group, the Moon Letters, is pronounced as /l/, and the following sound is not doubled. This rule is named like this not to give it a mystical, Middle-Eastern flavor, but for a logical reason: the word for sun, "شمس" 'shams,' starts with shiin, which is a Sun Letter. The word for moon, "قمر" 'qamar' begins with qaaf, a Moon Letter.
Luckily, this classification is not at all arbitrary, but phonetic.
The letters which are alveolar and/or dental are sun letters (except for jiim, ج): taa'(ت), thaa'(ث), daal (د), dhaal/zaal (ذ), raa'(ر), zaay (ز), siin (س), shiin (ش), Saad (ص), Daad (ض), Taa' (ط), DHaa'/Zaa' (خ), laam (ل), and nuun (ن).
And all the rest are Moon Letters: hamza seated on alif (أ/إ), baa' (ب), jiim/giim (ج), Haa' (ح), khaa' (خ), cayn (ع), ghayn (غ), faa' (ف), qaaf (ق), kaaf (ك), miim (م), haa' (ه), waaw (و), and yaa' (ي).