Había tanta lluvia que nos quedamos bajo la marquesina, junto al torniquete de la entrada.

Había is the imperfect form of haber, and here it gives background information: the rain was an ongoing situation.

So Había tanta lluvia... means There was so much rain... in the sense of a continuing condition.

If you used hubo, it would sound more like a completed event: there was in a more bounded, one-time sense. In this sentence, the speaker is setting the scene first and then giving the result: we stayed under the awning.

This is a very common structure in Spanish:

tanto/tanta/tantos/tantas + noun + que

It means:

so much / so many ... that

In this sentence:

tanta lluvia que nos quedamos...
= so much rain that we stayed...

Because lluvia is feminine singular, the form is tanta.

Compare:

• tanto ruido = so much noise
• tantas personas = so many people

Both are possible, but they focus on the idea slightly differently.

• Había tanta lluvia = there was so much rain
  
  • focuses on the amount of rain as a thing
• Llovía tanto = it was raining so much
  
  • focuses more on the action of raining

So the original sentence is perfectly natural, just a bit more noun-based in its wording.

Nos quedamos comes from quedarse, which here means to stay or to remain.

So:

• quedarse = to stay
• nos quedamos = we stayed

The pronoun nos is there because quedarse is a pronominal verb in this meaning.

In context, nos quedamos bajo la marquesina means: we stayed under the awning/canopy

Because it refers to the main action that happened as a result of the rain.

• había = background situation
• nos quedamos = the action taken in response

So the timeline is:

1. There was a lot of rain.
2. Because of that, we stayed under the awning.

If you said nos quedábamos, it would sound more like a repeated or ongoing habit, not a single reaction in that moment.

In Spain, marquesina often means a canopy, awning, or covered shelter.

In this sentence, bajo la marquesina most likely means under the awning/canopy near the entrance.

Important: this is a false friend for English speakers. It does not usually mean marquise in English.

Depending on context, marquesina can also refer to a bus shelter in Spain.

Both can be correct, but bajo is very natural and concise here.

• bajo la marquesina = under the awning
• debajo de la marquesina = underneath the awning

In many cases, bajo sounds a little more direct and idiomatic in this kind of description.

Because junto a means next to or beside, and when a is followed by el, they contract:

• a + el = al

So:

• junto a el torniquete → junto al torniquete

This is the same contraction you see in:

• voy al cine
• está al lado del banco

Here, torniquete means turnstile, the kind you pass through at an entrance.

Be careful: torniquete can also mean tourniquet in medical contexts. The correct meaning depends on the situation.

Because the sentence says de la entrada, the meaning here is clearly turnstile.

It specifies which turnstile:

• el torniquete = the turnstile
• el torniquete de la entrada = the entrance turnstile / the turnstile at the entrance

So de la entrada works like of the entrance or, more naturally in English, at the entrance.

Spanish often leaves out the article after quantity words like mucho, poco, tanto, etc.

So:

• mucha gente = a lot of people
• tanta lluvia = so much rain

You would not normally say la tanta lluvia here.

The sentence follows a very natural Spanish order:

Había tanta lluvia que nos quedamos bajo la marquesina, junto al torniquete de la entrada.

It goes from:

1. the situation
2. the consequence
3. the location

That makes it clear and idiomatic. Spanish could reorder parts for emphasis, but this version is straightforward and neutral.