Spanish and Crows.

I'm always thrown when I hear 'boa noite' in Portuguese as a greeting as I'm so conditioned to hearing it at the end of an evening in English.

#20
I need a day or two to tune in, to use bom dia (before noon) and boa tarde (from about 1 pm).

I was taught in school that día is masculine in gender: el día. Therefore it is buenos días, not buenas días. Both greetings are correct and in common use throughout the Spanish speaking world and mean the same: buen dia / buenos dias. For later in the day - good afternoon / evening (night): buenas tardes / buenas noches, or simply the informal "buenas".

Can't help with the Spanish, but with Q2.
In Dutch: hemelsbreed (heaven wide), so no references to flying objects

Some Adjectives Are Shortened When Used Before Certain Nouns

Spanish has about a dozen adjectives that are shortened sometimes when they appear before nouns.

The most common of these by far is uno, the number "one," which is usually translated as "a" or "an." See how it is shortened to un when it comes before a singular masculine noun: un muchacho ("a boy") but una muchacha ("a girl").

Here are seven other adjectives, all but the fifth one being quite common, that are shortened when they precede a singular masculine noun, as in these examples:

alguno ("some"): algún lugar ("some place)
bueno ("good"): el buen samaritano ("the good Samaritan")
malo ("bad"): este mal hombre ("this bad man")
ninguno ("no," "not one"): ningún perro ("no dog")
postrero ("last"): mi postrer adiós ("my last goodbye")
primero ("first"): primer encuentro ("first encounter")
tercero ("third"): Tercer Mundo ("Third World")
Note that the usual form is retained when the words are followed by feminine and/or plural nouns: algunos libros ("some books"), tercera mujer ("third woman").

The process of shortening these words is known by grammarians as apocopation.

A few other adjectives are apocopated under other circumstances:

Grande: The singular grande is shortened to gran before a noun in both the masculine and feminine. In that position, it usually means "great": un gran momento ("a great moment"), la gran explosión ("the great explosion"). However, grande is not apocopated when following más: el más grande escape ("the greatest escape"), el más grande americano ("the greatest American").

Cualquiera: When used as an adjective, cualquiera ("any" in the sense of "whatever") drops the -a before a noun: cualquier navegador ("any browser"), cualquier nivel ("whatever level").

Ciento: The word for "one hundred" is shortened before a noun or when used as part of a number that it multiples: cien dólares ("100 dollars"), cien millones ("100 million"). But it is not shortened within a number: ciento doce, ("112").

Santo: The title for a saint is shortened before the names of most males: San Diego ("St. James"), San Francisco ("St. Francis"). But the long form is retained if the following name begins with Do- or To-: Santo Domingo ("St. Dominic"), Santo Tomás ("St. Thomas").

Caiuss, I meant to say that it never replaces 'buenas noches' when saying good-bye.

#24, one should ever be so careful with the use of those words. For instance, 'uno' or 'una' can be quite despective depending on the situation and the person one's referring to.