In my grammar book I've encountered sentences where(according to the answers) there is possible both- present simple and present continuous, but I can understand only the use of pr. simple.
Could you explain, which use of continuous are these examples ?
Some languages mark tenses, aspects, and moods of verbs by inflecting (conjugating) the verb - by changing its suffix, for example. Some do it with extra words, often auxiliary verbs. English mostly uses auxiliary verbs, but inflects a little as well. So, person is shown with inflection - I go, you go, he/she/it goes. The inflection is simpler than in a lot of languages. We also indicate aspect with inflection, and that is what continuous (or progressive) forms do, usually by the addition of -ing.
The progressive aspect indicates that, whatever the verb is, it is or was happening continuously, at the time. Because it's the same in any tense (unlike the simple aspect, which inflects for tense - play/played, go/went), you have to have an auxiliary verb to indicate tense. That is generally the verb to be, so in the present tense, first person singular, it's "I am", and in the present tense, first person plural, it's "we are" - just like if you were "I am tall" or "we are bored", but instead of an adjective, you have the -ing form, in this case called a participle. The progressive aspect exists in other tenses as well, formed just the same - you can I can "I was going" or "I will be going".
Which you would use in different circumstances depends on the circumstances, but also on the verb itself. So, "I go swimming", which is would usually be used to assert that, at some point, going swimming is something you do. It might be used to contradict the statement "you don't go swimming" to assert that you do, in fact, go swimming sometimes. That meaning can be emphasised by added a further auxiliary verb, to do, as in "I do go swimming"; with the emphasis on "do", that is you asserting that you go swimming in contradiction to the claim that you do not. Alternatively, the simple present might be used with a qualification to indicate a specific time, such as "I go swimming on Fridays". You might even be asked, "what do you do on a Friday evening?" and reply "I go swimming", the specification of time being understood from the context of the question.
The progressive aspect indicates that, at the time in question, the action is actually happening - and usually that it is expected to stop happening at some point. So, while "I go swimming" merely asserts that there are times that you go swimming, "I am going swimming" asserts that you are doing it right now. Usually, that would mean that you are on your way to the pool, as being in the pool and swimming would be said as "I am swimming" - because to swim is also a verb, but it's being used as a noun, a construct called the gerund that exists in many languages. In English, it's just a little confusing that the gerund and the present participle, used to denote progressive aspect, are generally identical, being the -ing form. But that's a topic for another day.
In the full sentence of that example, the use of "always" means that it is not talking about any specific time, so the use of the progressive aspect does not fit. In the simple present, it suggests that you could go swimming at any time if the weather is suitable. In the progressive aspect, it just doesn't really work. However, "whether I am going swimming or not, I have to do my homework" does, because that says that homework must be done whether or not you are going swimming. In the example you gave, the condition, what determines the outcome, is the weather. In the case of homework, the condition is the question of whether you are going swimming.
In your second example, "we pass your house" suggests that there are times at which you (there being several of you) pass the other person's house. "We are passing your house" means that you are doing it right then. However, because this case is a clause that locates a statement in time, the progressive aspect doesn't mean you are doing it right now, but simply that there are times that you do it. As such, the two versions of the sentence are essentially equivalent, but the use of the simple aspect is more natural.
However, both cases are in each of your examples are grammatically and syntactically valid. They are well-constructed sentences (even if one does have a surplus comma). The first is semantically odd, that is it is hard to see its meaning without any context. However, it would be perfectly valid semantically and not seem strange in one context that I can immediately think of - in answer to the question "are you going [or coming] swimming?". Indeed, it would fit even better if there were an indication of time in the question, "are you going swimming tomorrow?". In that case, either answer is once again equivalent, and it would be normal to use the present tense even though you're talking about tomorrow, because that time specification is in the question. The questioner might naturally use the present or the future tense - the future being "will you be going swimming tomorrow" or "will you go swimming tomorrow", because, well, English is like that.
So, hopefully that exploration of what the progressive versus simple aspect means will help you understand generally, but the simple answer is that both of your examples are, depending on context, valid in either simple or progressive aspect. Because of the complex nature of the sentences, with multiple clauses, there is no essential difference in meaning between the two, even though there is usually a difference in meaning between the aspects. I fear that I have made a very complex answer to end up at a simple conclusion, but without better knowing how well you understand simpler English grammar, I can't be sure which bits I can skip over.
