Raining

Raining
 

If I want to say "It is raining so bring an umbrella" would I say: Está lloviendo tan trae un paraguas?

"Tan" is a comparative, not the word you need here. Take a look at your dictionary and check other meanings of "so". :)

así que? or just así?

"Así que". :)

it's raining = llueve
it's raining (right now) = está lloviendo
so = así que

So if I was talking about the future would I say: El lunes lloverá OR El lunes estará lloviendo

The future can be described a few different ways in Spanish, but the future progressive form isn't one of them.

1) The Spanish simple present tense can be used to express the immediate future.

It'll rain this afternoon. = Llueve esta tarde.
We're leaving for Mexico tomorrow. = Salimos para México mañana.


2) The 'ir' + 'a' + infinitive form is a very common way to express the future.

It'll rain on Monday (so says the forecast). = Va a llover el lunes. (It's going to rain on Monday.)


3) Finally, the simple future tense can be used to express the future, but it is often used
for unplanned events (date not mentioned, some unpredictability suggested).

If you ask a question using the simple future tense, it's like saying 'I wonder if it will ___' or probability, 'It'll probably ___'.

I wonder if it'll rain on Monday. = ¿Lloverá el lunes?