For the original sentence, could it be a realistic posibility to use "Tago malvarmas" since "tago" is already a singular indefinite noun and "malvarmas" is a verb form of the adjective "malvarma". I mean this in the purposes of speech, as in is it natural sounding or not.
When you say "ĝi estas malvarma" en Esperanto, "ĝi" (it) refers to something specific which has already been named, e.g. the coffee, the water, the day, the room. In that case the word "malvarma" (cold) correctly has an adjective ending. But in English we also like to say things like "It's cold", "it's dark", "it's unfortunate that..." where "it" doesn't refer to anything specific, but is simply a way of introducing the phrase. In Esperanto, where there's no specific subject in expressions like that, you just say "estas..." and follow it up with an adverb, not an adjective: "estas malvarme", "estas mallume", "estas bedaŭrinde ke..." Adverbs are used a great deal in Esperanto, as you will discover. I hope you can follow this very brief explanation of a point which I suppose will come later in the course.
I made that mistake also, so I looked it up.
"today " is "hodiaŭ"
Here are some dictionaries that I am finding useful: http://www.gutenberg.org/files/16967/16967-h/16967-h.htm (English to Esperanto)
http://esperanto-panorama.net/vortaro/eoen.htm (Esperanto to English)
I looked up "small" and the dictionary still gave me "malgranda", but in another discussion someone gave the word for "tiny" as "eta" so I looked up "tiny", but what the dictionary gave me was "malgrandeta" so I must have misunderstood the person who was probably trying to explain the ending change for "tiny", but just to make sure I looked up "eta" and lo and behold it is listed as "tiny" - maybe it is short for the other one. It is an exception.
Even the word for "a dwarf" which is "malgrandegulo" is the opposite of the word for "a giant".
"Cold" came up as ""malvarma", but "to catch a cold" or "have a cold" use different words. "Frigid is "glaciiga" and "cool" is "malvarmeta"
Here see what you can find! If you go to a different intensity, there is usually another word; otherwise, it looks as though there would be no need for a thesaurus in Esperanto.
HERE IS ANOTHER DICTIONARY: http://www.archive.org/stream/englishesperanto00oconuoft/englishesperanto00oconuoft_djvu.txt
I'd say you're 99% good there. The only thing I'd clarify is that affixes stack in an established sequence.
So you start with the root, whether "varm-" or "dorm-" or what have you, and then add the "-et-" suffix to indicate smallness, and then add either "-o" to indicate noun or "-a" to indicate adjective (or "-e" to indicate adverb, etc.)
And then you might add the suffix "-j" to indicate plural, and then you might add the suffix "-n" to indicate the accusative.
Ye-es, but it's not what you'd expect. The word "ĝi" has to refer to something. So, "Ĝi estas malvarma tago.", needs something to refer to: "Kiu estas malvarma tago?" (Which is a cold day?) "Tiu estas malvarma tago." (That is a cold day!)
(Barring errors:) Imagine that you've kept all the days of your tear-off calenders in a box. Every day you wrote the temperature beneath the date, thus you now have years of days with temperatures. Today, together with your friend, you're going to sort all the days by temperature into to piles: One pile is "varma tagoj", above 18°C; the other pile is "malvarma tagoj", below 18°C. But your friend asks: "Kie ni metas tagon de 18 gradoj." (Where do we put a day of 18 degrees?) You think about it, and decide: "Ĝi estas malvarma tago." (It is a cold day.)
Ah, but it's actually about "malvarma": cold. That makes it all a bit topsy-turvy, but let me try:
It's summer, and during the day the air-conditioning in the office-building is at full blast. During the night the temperature rises significantly because it's still warm outside, while inside the air-conditioning is off. A cat with her litter live inside the building in the spaces where humans don't go. They can't see the outside; all they experience are the temperature changes in the building. One of the young asks: "Panjo, kia estas la diferenco inter tago kaj nokto?" (Mum, what is the difference between day and night?) And the mother answers: "Tago estas malvarma." (A day is cold.)
You could think of it that way, though there is no indefinite article in Esperanto, so it is really everything other than the definite article. Unfortunately, I believe the definite article may be used, as in Romance languages, with some general nouns that English would use a zero article with as well. Generally, though, "the" in English will be "la" in Esperanto and "a, an" or zero article in English would simply lack the article in Esperanto, as here.
Esperanto is not a Germano-Romance language. Zamenhoff incorporated vocabulary and grammar bits from many different European languages, including Germanic and Romance, yes, but also Slavic. But that does not make Esperanto a Germano-Romance-Slavic language. Esperanto is only inspired by these languages, but it is not categorized as part of any of those families.
Dr Zamenhof certainly intended for his language not to be Romance or Germanic, but beyond the word for sausage (unavoidable, I suppose, in someone living in Poland), where do you see any influence of a Slavic language, particularly in Esperanto grammar?
Actually, I did find one online that I had not thought of, the Esperanto cxu coming from the Polish czy. That sort of question word is certainly not found in Germanic or Romance languages. Indeed, I'm not sure I remember it being in any Slavic language other than Polish, either.
The article "la" is used both with separate nouns and with nouns with an adjective in front of them. It's used for specific instances:
"Bona seĝo multekostas." (A good chair is expensive.) This is about any good chair, which are expensive.
"La bona seĝo multekostas." (The good chair is expensive.) This is about a particular chair, which is good and expensive.
The counter-intuitive part for people used to a Germanic or Romance language is that there's no indefinite article. Had there been one, the difference between the sentences with "la" and those without "la" would have been obvious.