Now, we have lots of resources for learning Japanese - tons of textbooks, dictionaries, audio and video recordings, educational software etc.
But in the past - How did Europeans first approach Japanese language? (They didn't have any of these resources..)
How did they start learning it for the first time?
When the Jesuits first came to Japan, they needed a word for God. They described the qualities of God to local priests, and the priests came back with 大日様. When the Jesuits went around preaching 大日様, though, they were concerned to find that the Shingon monks seemed unusually happy about this, and eventually learned that they had chosen a sectarian term. They wound up going with 神 which is just as bad.
Generally the Jesuits would learn the grammar of a language as they wrote down vocabulary. Project Gutenberg has a Portugese-Latin-Japanese grammar which may interest you. The attempt to fit Japanese into Latin grammatical terms is a little clumsy, but the problems they dealt with are exactly the same as ours today:
There are two ways to count in Japanese. The first is with the ordinary numerals which are called iomi. With these one is able to count to ten; e.g., fitotçu means 'one,' which is also used to say 'a little,' as in saqe fitotçu nomaxite tamǒre 'give me a little sake to drink.' Futatçu means 'two,' mitçu 'three,' iotçu 'four,' itçutçu 'five,' mutçu 'six,' nanatçu 'seven,' iatçu 'eight,' coconotçu 'nine,' and tovo 'ten.' Icutçu means 'what?' and is used when one does not have the proper number.
The second way of counting is with the coie vocables which are borrowed from Chinese. These numbers are not used by themselves to count to ten; but are rather used when counting things which are represented by Chinese, and not Japanese vocables. These bound numerals (termini numerales) are: ichi 'one,' ni 'two,' san 'three,' xi 'four,' go 'five,' rocu 'six,' xichi 'seven,' fachi 'eight,' cu 'nine,' jú 'ten.' The numbers eleven and above are made by joining these numbers together. Thus, 'eleven' is jǔichi; júni is 'twelve,' júsan 'thirteen,' júcu 'ninteen.' The tens are obtained by placing one of the numbers in front of ten; e.g., nijú 'twenty,' sanjú 'thirty,' sanjǔichi 'thirty-one,' cujǔ 'ninety.' Fiacu means 'hundred,' fiacu ichi 'one hundred and one,' fiacu jǔ 'one hundred and ten,' fiacu sanjǔ 'one hundred and thirty,' ni fiacu 'two hundred,' sambiacu 'three hundred.' Xen means 'thousand,' and xen roppiacu sanjǔ ichi is 'sixteen thirty-one.'
When enumerating sermons, homilies (tractatus), or repetitions of things, fen is placed after the numeral; e.g., ippen 'one sermon,' nifen 'two,' sanben 'three,' ave maria fiacu gojippen 'one hundred and fifty Hail Mary's.'
The enumeration of masses and congregations of men is done by placing za after the numeral; e.g., ichiza 'one congregation,' niza 'two,' sanza 'three,' jǔza, or better toza 'ten.'
And so on for many lines.
