Spanish language Q&A Forum

I have seen a lot of sentences like the examples below:

1. Todavía no han llegado, su avión debe haberse retrasado.

They haven't arrived yet, their flight must have had a delay.

2. Cristina ha debido de ser una buena profesora, sus estudiantes le han comprado flores.

Cristina must have been a good teacher, her students bought her some flowers.

I respect that Spanish uses punctuation differently, in some cases, from the way English uses punctuation.  However, the Spanish sentences and the English translations use a comma to separate the two sentences in each example (these examples were taken from a quiz on Kwiziq).  For the Spanish, I've checked RAE and I cannot understand why these two sentences are joined by a comma when it seems they should be separated by a period or a semicolon (or even possibly adding a connector or conjunction to join them).  For the English translation, in American English we would have to somehow separate these two complete sentences with some form of punctuation (period or semicolon).  I have also seen similar constructions in other writing, but not usually in newspapers or academic writing.  If you could provide an explanation, I would appreciate it. Thank you.

Hola Shui, Inma y Anna,

¡Estoy tan feliz con este reader! 

Muchísimas gracias

Wouldn't we always use "el azúcar" for phonetic reasons?

Thanks!

Marcos

Can I use it without and with que interchangeably?

Instead of just adding "Me llamo", I wrote "me llamo Juan" inadvertently giving the answer "Me llamo Juan Juan". This is accidental! Yet Kwiziq marks it incorrect! Almost certainly, this would never occur in a practical situation.

Also when using my tablet, I sometimes miss out a question by accident. Kwiziq should prompt that all the questions have not been answered. This is also accidental! Yet Kwiziq marks it incorrect! 

Is there a reason why "and you can see advertising" translates as "es posible encontrar publicidad." Would "y [se] puede ver publicidad be acceptable? 

 Since the preposition “a” as a personal “a” wouldn’t “les” also be a correct?

How many distinct classes of irregular verbs are there in the subjunctive present tense?
If it is possible to say, how many distinct irregular verbs that do not belong to a class, such as ir, are there in the subjunctive present tense?
Knowing the answers to these questions could help me form a strategy to learn them most effectively and efficiently. I would love to simplify the learning of this topic.

Thank you, James

In the examples we have “llegar a casa”, “salgo de casa”, and there is also the expression “estoy en casa”.  None of these expressions use articles. They also all use verbs of movement or location. 

Do we ever use the definite article with “casa”, for example, to say “I’m going back to the house” by saying “Regreso a la casa”? Or does it change the English translation if we omit the definite article, that is, if we say “Regreso a casa” does it mean “I’m going back home”? 

I know this is off topic but you use many rich examples that provoke questions.