Because "dies-" is a "Der Word". Der words takes the definite article ending. Since baby is neutral and the definite article for Nominative (and accusative) is "das", it takes the "s" ending. Das Baby: Dieses Baby. This same rule is also very similar to adjective endings. Here is a list of Der and Ein words and how to use them. The 2nd link is how to use adjective endings which work in a very similar fashion. These both take a while to get down so spend a few weeks on both of these and YouTube is also very insightful (and free).
No, not always.
Predicate adjectives (roughly: after the verb "to be") have no gender/number/case ending at all:
- Der Mann ist glücklich. (The man is happy)
- Die Frau ist glücklich. (The woman is happy)
- Das Kind ist glücklich. (The child is happy)
- Die Menschen sind glücklich. (The people are happy)
Attributive adjectives (before a noun) always have an ending -- but that ending distinctly shows the gender (and number and case) only if there is no determiner (such as an article) before the adjective: guter Wein, gute Sahne, gutes Wasser, gute Getränke "good wine, good cream, good water, good drinks". (This is "strong inflection".)
But if there is a definite article before the adjective, the adjectives get a more neutral -e or -en ending, regardless of which gender the noun is: der glückliche Mann, die glückliche Frau, das glückliche Kind, die glücklichen Menschen. (This is "weak inflection".)
And if there is an indefinite article, the adjectives usually have neutral -e(n) endings, but in the nominative and accusative singular, they do show the gender, because the article does not do so: ein glücklicher Mann, eine glückliche Frau, ein glückliches Kind. (This is "mixed inflection".)
sorry I don't undestand, why is it schwarze instead of schwarzen? if haare is plurar, according to this article it should be schwarzen :/ http://www.nthuleen.com/teach/grammar/adjektivendungenexpl.html
There is no determiner before the adjective (no article, possessive, etc.), so it takes strong inflection, not weak or mixed.
- schwarze Haare (strong ending -e)
- keine schwarzen Haare (mixed ending -en)
- die schwarzen Haare (weak ending -en)
Since (die) Haare is plural, why isn't it "schwarzen"? I couldn't find a mention about this on the class' tips (https://www.duolingo.com/skill/de/Colors/tips-and-notes)