I will start with the questions.
1. This issue is actually pretty simple, however the translation in English is often not sufficient as there is no gender and case difference in this language...
"Oni" and "one" both mean "they", but "oni" is the masculine version and "one" is feminine. I think it is the same as "eles" and "elas" in Portuguese. So:
Oni są wysocy = Eles são altos
One są wysokie = Elas são altas
Oni są niscy = Eles são baixos
One są niskie = Elas são baixas
You can notice that the adjective changes according to gender as well.
2. Oooh, now it's the funny part - cases :)
"Polska" is the basic version of noun and we use it when it's the main noun of the sentence. For example, if you want to say "Poland is awesome" it will be: Polska jest wspaniała.
But when you intend to say that you live somewhere, for the noun that stands for the place you have to use the case of location (it's called locative in English and locativo in Portuguese). Then I live in Poland = Mieszkam w Polsce.
It also applies to many kind of talking about location that you can imagine, for example:
I am in Poland = Jestem w Polsce (BUT I am from Poland = Jestem z Polski, that's a different case)
[ I will write more examples later... now I'm not really creative ]
Ok, let's go to your excercises.
Jestem z
Cases again... This time it's a different one, it's called genitive and honestly I don't know how to explain it properly not to mess everything up... Let's say for now that one of applications occurs for the nouns where you would put "de" in front in Portuguese:P It's for places of origin and describing belongings etc. There should be: Jestem z Brazylii.
Language purist here :P I would say "to moje drugie ćwiczenie" (ćwiczenie means excercise), because it's sound better in this meaning. The word "praca" means also job, so your sentence could also mean "this is my second job" which is confusing. Of course it's not a mistake, just a detail:)
As you can see the adjective changed here as well, it's because the gender of the noun changed. "Praca" is feminine so it used the feminine adjective "druga". "Ćwiczenie" is the neutral gender, so the adjective is "drugie". (For the masculine it would be "drugi"; plural masculine - "drudzy", plural feminine - "drugie". In Portuguese adjectives also change depending on the gender of the noun, right? So it's very similar here, we just have one more gender because we're crazy :P )
Jestem mężczyzną,
Another case of case. And you used it ALMOST properly. This one is called instrumental case(?). I dunno, I need to learn it first :P But. "Brunetami" is plural and there is only one you so it's better to use singular - it should be : Jestem brunetem.
Nie jestem wolny, lub żonaty, bądź rozwiedziony, ale kocham
Guess what? Cases! This one's name is accusative. This sentence should look like : Kocham moją sympatię.
In addition, "dziewczyna" is a better and more common word for a girlfriend. It's the same word as for girl, so basically "my girlfriend" is the same as "my girl" - moja dziewczyna. So : I love my girlfriend = Kocham moją dziewczynę.
"Sympatia" generally means girlfriend too, but it's used only by people who are about 200 years old.
"Żona" should also look like "żonę" here. But, well, this is more abstract case :P
Ona jest
The same issue as before, you used the right case (instrumental), but in plural. I'm not that fat and there is no need to use plural for me :P so it should be : Ona jest blondynką.
(I would ignore the fact that you lie in your writing excercises... Or you can't recognize colors)
Here I should get really really angry. Why the hell you used masculine adjectives to describe me? :(
There should be : szczupła, piękna because I'm a woman and female version suits me better.
Gender, dude. As far as I know, your mother is a woman, so there should be : moja
You forgot the verb: your mother has the name.
I'm not sure I know what you meant here.
If you meant "she is old" --- ona jest stara.
If you meant "they are old" --- oni są starzy.
Anyway - you're so nice!
Jestem
The same, instrumental case. I will explain the theory later... Plus I would put the words in different order. So : Jestem pierwszym i jedynym synem
This is a simple mistake :) This is "I am" - "Ja jestem", I'm sure it's just an accidental mistake as you used the verb properly in the first sentence.
And somehow you used the instrumental case properly in this sentence - Jestem jedynakiem :)
Gosh no more colors :(
Mój ojciec ma jedną córkę, ale ona nie brat studiuje i ja nie
Again you forgot the verb :)
Here I have no idea what you tried to say...
Double combination, past tense and a case. First, the past tense for "I met" is "ja poznałem". The case here is genitive : "moja siostra" changes to "mojej siostry".
The whole sentence: I never met my sister = Nigdy nie poznałem mojej siostry.
This is it. I have to say that it's really good for some first writing in a totally new language, I'm impressed :) Everything will come with time.
The main problem with busuu is that it makes moreless the same content for every language, without concentrating on the specific issues of every language. So I think it would be really really useful for you to ask about everything you doubt :) My advice is to learn the whole sentences or expressions, to memorize and get used to the words in specific situations. Also when you check some vocabulary it would be better to search for whole expression/sentence in Google translate instead. Of course it's not always 100% correct but... well.
I think it would be nice for you to know something about cases, right? In the weekend I will try to search for some useful information what is that shit and when to use what. Maybe check the book I sent you, I don't remember if there is something there.
Okaaay, it's really late now. Good night/Good morning, have a nice day, I hope my advice helped you a bit.
