No, "always" is not correct. It depends on the language.
Some languages have a distinction between present simple and present progressive that is similar to English; Irish and Turkish come to mind. In such a case, the correct tense has to be chosen, in general.
Many of the languages taught on Duolingo do not make a distinction, and then the present tense could correspond to either in English, that is true.
Since there is a huge difference between 'I am eating' and 'I eat', I think it is important that each tense is translated to the proper, corresponding tense, or people will be taught that the difference is neglectable. Learn to spot the difference instead and stop trying to find shortcuts. Every aspect of the languages deserves to be preserved and used.
al= a+il eg.: al cinema=to the cinema; ai= a+i eg.: ai ragazzi=to the children. You can learn the prepositions if you search 'le preposizioni' ;)
And an interesting fact: italians use the masculine noun even if there are many girls and only one boy. So I think that's why ragazzi can mean children and boys also.
bambini and ragazzi can both mean children. However, bambini almost always refers exclusively to young children. Ragazzi typically refers to teenagers or youths, in the sense of "children". Therefore, if you have to translate bambini, use the word "children". If you have to translate ragazzi or ragazze, "children" might or might not work, depending on the context. It is safer to stick to "boys" or "girls".
In a more colloquial sense, ragazzi (mixed group or m.pl) and ragazze (f. pl) can be used to talk to / about close adult friends, just as "girls", "boys" or "guys" can be used in English.
il mio ragazzo can also be used to mean "my boyfriend"
il bimbo / la bimba are words that you may also find used in reference to very young children
When parents say that they have 2 children, they generally use the term filgi (boys or mixed) or figlie (just girls), e.g. abbiamo due figli. These nouns are il figlio and la figlia in the singular.
(Warning for Italian native speakers: be very careful about using the word "bimbo" in English - it has a very different meaning! http://www.collinsdictionary.com/dictionary/english-italian/bimbo )
These may help:
I hope this helps.
Io scrivo, ai (a+i), ragazzi. =
I write, to the, boys/children
Quick, write the children a letter! =
Svelto, scrivi una lettera ai ragazzi!
I wrote (to) my grandmother yesterday to ask whether or not she likes her new yoga class. =
Ieri ho scritto a mia nonna per chiedere se le piace la sua nuova lezione di yoga.
The basic preposition is "a" (to, at).
"al" is already a merged form of "a + il" (with the masculine singular definite article), and "ai" is a merged form of "a + i" (with the masculine plural definite article).
See https://en.wiktionary.org/wiki/a#Italian for a list of more merged forms of "a" with other forms of the definite article.
In italian everything is either female or male, - there is no neutrum.
Un bambino = a baby boy - / - una bambina = a baby girl
Due bambini = two baby boys - / - due bambine = two baby girls
Un ragazzo = a boy - / - una ragazza = a girl
Due ragazzi = two boys - / - due ragazze = two girls
But if you want to refer to a mixed group of children, e.g. 2 girls and 2 boys, you use the male plural form. Depending on their age it's either "i bambini" (if they are small children) or "i ragazzi" (if they are a bit older children).
So without more context there is really no way to know if ". .ai ragzzi." is referring to a group of boys or a mixed group of children.