When do you use these structures?
I am sorry that I have so many questions!
I know that "so am I" and "so I am" are both things that can be used in different context but I don't understand the difference between them and when to use each.
I also don't know if there is a difference between saying "like" and "as if". For example: It looks like it's going to be a sunny day. It looks as if it's going to be a sunny day. Are both correct?
And finally, my last question. When is the subjunctive used in English? What I understand is that the subjunctive is used when we want to emphasize something important. For example: It is important that he arrive on time. Or when we want to suggest something: I suggest that she visit the doctor. But I heard the subjunctive is used in other cases with different structures. For example in the second conditional when you say: I wish I were good at swimming. And I think I understand this structures because they are hypothetical things. But why is this subjunctive? "It's time we left." Doesn't that mean that we are leaving now? Are there any other cases of subjunctive in English?
Edit: Why do we use it for suggestions? it's not a hypothetical situation either
Thanks!
2 Answers
To answer your last question: When is the subjunctive used in English?
This question depends heavily on the local dialect (for example, I pretty much never use it). There are three major uses of the subjunctive in English, so I'll break down each one and indicate where to use each.
Present Subjunctive
The first similar to the Spanish subjunctive, and occurs when the dependent clause is in the present tense and the independent clause contains a verb that "triggers" the subjunctive. There are very few of such verbs.
It is necessary that he **go** outside
The issue is that this use of the subjunctive is pretty much restricted to formal English. Most Americans (I'm not sure about other countries) , would just say "It is necessary that he goes outside".
Honestly, I wouldn't worry about this. So few people (at least in the US) talk like this, that it's not worth fretting about this use.
Conditionals
In Spanish, for counter-to-fact conditionals, you use the subjunctive. (Compare "Si yo fuera rico" with "Si yo voy a la escuela"). In Standard English, you use the past tense of the subjunctive ("were') when introducing these conditionals. So you'd say "If I were rich", but "If I go to school". Basically, if you'd use el imperfecto del subjuntivo for the verbs "ser" or "estar", simply use the word "were" in English.
Similarly, for verbs that are not "to be", you can form the conditional with "were to". For example "If I were to dance". However, it's more common to just use the preterit and say "If I danced".
On that note, in some parts of the US (including where I live), this use of the subjunctive is not used. I'd instead say "If I was rich", instead of the Standard "If I were rich".
If I was you, I'd use this subjunctive since it's a part of Standard English. Just be aware that many Americans will not employ it.
Set Phrases
There are a few set phrases that still employ the subjunctive. These should be learned by heart. Examples include: "Come Friday, we'll go outside" and "The powers that be".
Hello Uvasol
Wow what a lot of questions!
We are very pleased that you are so keen (enthusiastic) to learn...and such a desire to learn is admirable! 
Please never apologise for wanting to ask questions so that you can learn...it's good to want to learn and understand subjects better!
I would simply suggest that you perhaps ask just two or three related questions at one time in future to allow people to give you more detailed answers and explain things more clearly! If you wnat to ask more than 3 questions at one time, just post another question. separately!
You asked:
I am sorry that I have so many questions! I know that "so am I" and "so I am" are both things that can be used in different context but I don't understand the difference between them and when to use each.
Firstly, let's deal with the difference between so am I and so I am
So am I vs So I am
So am I Context 1
ex 1 Woman: What (work) do you do?
Man: "I am a teacher!"
Woman: "Wow, so am I!" (meaning: I am also a teacher/I am a teacher, too!)
ex 2 girl: "I am happy because you are my friend!"
Boy: "So am I" (context: I am also happy for the same reason)
Context The words: "So (therefore) I am ......"are not usually spoken by themselves (in isolation ) but in the context of a complete sentence. If somebody said the words to me: "So I am" I would reply So am I, what? So am I hungry, so am I tired, so am I going shopping?
ex 1 Somebody might say:" My car broke down yesterday so (meaning: therefore/for this reason) I am going to have to catch the bus to work!" (to travel by bus)
ex 2 woman: "My husband lost his job yesterday, so I am going to have to look for a job!"
ex 3 Teacher: "The sun has just come out so I am certain the rain will stop and the weather will improve!"
I will continue to answer your questions:
I also don't know if there is a difference between saying "like" and "as if". For example: It looks like it's going to be a sunny day. It looks as if it's going to be a sunny day. Are both correct?
Looks like vs looks as if They both really mean the same thing ...I would say the first is more common.
It looks like Sally is going to get married, at last! /It looks as if Sally is going to get married at last!
And finally, my last question. When is the subjunctive used in English? What I understand is that the subjunctive is used when we want to emphasize something important. For example: It is important that he arrive on time. Or when we want to suggest something: I suggest that she visit the doctor.
The subjunctive is not often used in English , certainly not as frequently as the Spanish use it!
The subjunctive is used to make hypothetical statements: They commonly start with the highlighted words: if I were /If you were
eg: If I were to win the lottery, I would donate half my winnings to help orphan children.
If you were to have an accident, who would look after (care for) your children
You could say also rephrase the first part without using the subjunctive: If you had an accident ...etc....
Like in the Spanish, the conditional (indicative) tense is used in combination with the subjunctive
I believe that it is also correct to say that it can be used when you want to ask somebody to do something
I am not claiming to be an expert on English grammar! In fact, it is true to say, that my parents' generation received a much better (solid) grounding / foundational teaching in grammar at school than I did, or many successive generations of students later! I have learnt more about English grammar through learning foreign languages than I did at school. I am sure that Ian Hill and others will have a better understanding of English grammar than I do! (If I am not mistaken, Ian has taught English as a foreign language!)
The problem is that in England we are often taught something without any explanation or clear understanding why even need to know that information, or what relevance it has for our lives now or in the future! Therefore many kids just 'switch off' (stop listeneing) in schools, thinking: what use is that knowledge to me?!
I hope this helps 
