How did ancient foreigners learn Chinese?

This is a big problem, involving phonetics, cognitive linguistics and first language acquisition. ......

Pick the simplest one. The ancients had a method called "anti-cutting" for phonetic notation, such as the red cutting of East and Germany. It means that Dongzi's sound is a combination of the initial consonant of German and the vowel of the red letter with tone. Come again, lai, change and cut.