Should linguists speak more than 1 language? (P.2)

I've just read chapter 10 "Strukturer i målspråk og morsmål" in "Med språklige minoriteter i klassen. Språklige og faglige utfordringer" by Kirsti MacDonald and Ryen Else in my kompedium, in which the Vietnamese language is mentioned a few times, with a few words on the structure, syntax... 
There are some silly errors.
1/ People seem to have a mistaken view (which I've encountered a few times) that as the Vietnamese language lacks inflections, there must be function words to denote tense such as đã for the past and sẽ for the future.
As a native speaker, I do not know how Vietnamese is taught as an 2nd or foreign language, but I would say that we rarely say đã or sẽ and sentences with either of these 2 words used superfluously sound weird to us. Let me clarify. Sẽ can be used for the future tense, as in Tôi sẽ nói với ông ấy (I will tell him) or Con trai tôi sẽ lấy vợ vào đầu năm sau (My son is going to get married at the beginning of next year) or Thằng đấy sẽ chết sớm nếu cứ hủy hoại bản thân như thế (That guy will soon die if he keeps destroying his own body that way). But it isn't always compulsory. We can say Mai tao làm (Tomorrow I'll do it) or Năm sau anh đi Úc (I'm going to go to Australia next year) or Tuần sau nó qua chơi Oslo (Next week he/she is visiting Oslo). In other words, as far as I know, sẽ can be dropped when there are adverbials denoting the future tense.
Saying Mai tao sẽ làm, Năm sau anh sẽ đi Úc and Tuần sau nó sẽ qua chơi Oslo is not very natural but OK. 
Đã is slightly different. We can say Hôm qua tao đi bộ về nhà (Yesterday I walked home) or Con làm bài tập tối qua (I did my homework yesterday evening) or Chị tớ tốt nghiệp đại học 2 năm trước (My sister graduated from university 2 years ago), without đã. The word đã is used to emphasise that something has been done, finished, which means it is acceptable in the 2nd and the 3rd sentences (and they are better translated as I already did my homework yesterday evening and My sister already graduated from university 2 years ago), but no one says Hôm qua tao đã đi bộ về nhà. In some sentences, đã denotes the present perfect, instead of the past tense. For instance, Con đã làm bài tập (I have done my homework). Sometimes đã can be replaced with rồi, though it is also present perfect- Mình bảo rồi (I've told you), and if it's past tense, rồi also emphasises the fact that something has been done, as in Con làm bài tập tối qua rồi (I already did my homework yesterday evening).
Which means that the example in the book is wrong. Tim reiste cannot be translated as Tim đã đi.

2/ The book says hus (house) is nhà and huset (the house) is nhà ây.
The 1st issue is ây. Apparently they mean ấy. The Vietnamese language has 5 diacritics and these tiny things make all the difference.
The 2nd issue is that ấy is not used very commonly as an equivalent of the, this, that... in English. We may say cô ấy, bà ấy, chị ấy, anh ấy, ông ấy, lão ấy... but these words would be translated as she or he. We may say chuyện ấy, but usually it's better translated as it, this or that, as in Chuyện ấy không liên quan gì tới anh (This doesn't have anything to do with you), though very often Vietnamese people use the word chuyện ấy to refer to sex, and cái ấy (literal: that thing) to refer to penis. The word ấy may be used for time, such as năm ấy (that year), ngày ấy (that day), etc. But I definitely don't use ấy for things such as tables, desks, pens, bottles, pillows, laptops, phones, wardrobes, TVs... (basically the objects around me now). Đó, for instance, is a better word.
The 3rd issue is that huset is definitely not translated as nhà ấy, but not as nhà đó either. There must be a word before it, such as căn, ngôi or cái. Nhà, without any word preceding, very often means home, family, husband or wife. The same goes with other things, there must always be some word before it, such as con for animals, in general, or cái/ chiếc for things, in general, though it's more complex and diverse.
etc. etc.


See the danger of not speaking many languages or of speaking languages that are not very different, linguists?
See the danger of talking about a language one doesn't actually know?