Do you use a question mark when a sentence starts as a question but then turns into a statement? For example, "Can you explain this to me, because I don't understand the second part." Another example would be, "What did he just say, because I couldn't hear him over the noise of the television." I usually use a period but don't know if that is correct.
It's allowed, but mostly inadvisable.
The times it is most allowable are:
You have a series of related questions:
Do you know what he was called? where he was from? who is people are?
Expressing uncertainty within a sentence:
This feeling, this anger? rage? fury? I am over-whelmed.
You are living in the 18th Century. (It used to be more common than now).
It can be confusing though. It's often a good idea to rephrase, especially in formal text because of this potential for confusion. I could easily rephrase my first example just by capitalising. Your example questions can be simply broken into sentences such as:
Can you explain this to me? I don't understand the second part.
Or expressed as a single question with a subordinate clause:
Can you explain this to me, because I don't understand the second part?
I don't understand the second part; can you explain this to me?
My second example can't be re-ordered without losing the effect. This effect would only be suitable in some sort of writing anyway, and the choice is between keeping the risk of confusion, or losing the effect.
(And as for the spaces: Two spaces if you are writing on a manual typewriter something that you intend to be typeset professionally if published. One for all other writing. Typesetting, graphic use of text, and other cases where you're going to pay attention to the weight, block-size, kerning, extender-size relative to leading, etc. would include spacing in a variety of different cases as part of those considerations, but usually would be a space slightly larger than a "single space").
The question mark is designed to clarify for readers of text that something is a 'question' rather than a 'statement', i.e. that it commands an answer. So it is a functional item, not a structural one. If you are expecting an answer, you indicate this with a question mark, even if the sentence is framed as a statement, or changes throughout it. Clarity can be added by the use of parentheses to highlight a 'question element', but it is the intent of the author to indicate a answer-inviting string of text that makes it a question, and to clarify that is the case and resolve ambiguity, a question mark is applied.
It is far clearer how this works by analogy. Consider the punctuation mark for 'irony':
By definition being ironic is saying one thing and meaning the other. Whilst satirical writers often want to deliberately confuse issues, for comic or expository effect, if you were to write a play and want to make clear for the purposes of direction that a thing is said 'ironically' you could use the mark to indicate this to aid understanding for those concerned. It may well be the actor wishes to convey it in a more intentional, non-ironic way, or it might suggest ways for the actor to interpret that part of the play, but if there is a need for it to be unambiguously ironic to at least some group, then the use of the irony mark serves a particular functional purpose in doing that.
(Wikipedia reference provided as a 'gentle introduction' to the topic of irony marks, but there is plenty of discussion about this online)