When I first learnt the numbers, I learnt
一 with the first tone
But now, during a Memrise lesson, they teach me that
一个 should be pronounced
yi2 ge4 or
Is this correct and, if so, is there a reason for that pronunciation?
一 is pronounced in the first tone when it stands alone. It is pronounced in the fourth tone when it precedes a first, second, or third tone. However, it is pronounced in the second tone when it precedes a fourth tone. 不 is a bit similar: It is also pronounced in the fourth tone when it precedes a first, second, or third tone. However, it is pronounced in the second tone when it precedes a fourth tone.
Some examples (from: http://web.mit.edu/jinzhang/www/pinyin/tones/index.html#1bu but I used simplified characters instead and corrected a tone which is incorrect on that website)
Since 个 has a fourth tone, the tone of 一 should become a second tone. Although in practice 个 is most often spoken as a neutral tone, the original tone is used for the tone sandhi.
YI2 GE4 or yi2 ge5 are both correct, and it depends on where you using them. When we use 一 and a quantifier together, we always proniunce 一 as Yi2, and there is no specific reason.