Practice of A TOEIC Test

　I think that this is more natural. Also, we should use "the" instead of "a" because I believe that you are referring to the upcoming TOEIC test. Recall that we use "a/an" for non-specific things, and "the" for specific things. We can actually use "a/an" when referring to a single TOEIC past test, because there are many of them. However, when we are referring to a specific single TOEIC past test, such as the 2012 TOEIC past test (I don't know if one really exists), then we use "the". :)

　We use "solving" instead of "solve" here because "solve" is used when we are talking about the future, or in general, such as in "I will solve" and "we're going to find out who it was and solve the mystery" respectively. "Solving" is used when we are talking about the present.

(Remember that when the events described in a sentence actually happened don't really have to do with deciding between present-tense/past-tense/future-tense, or words that deal with the present/past/future... it depends on the "time-sphere" of the sentence.
Here, we are describing you solving the problems as if it were happening right now, when in fact you did it some time before writing the lang-8 journal entry.)

"Assuming" is a conjunction where the idea before it is used as a premise for which the statement coming after it can be based. It's also used for arguments, which are a set of reasons which hope to persuade someone about a topic, so we can't use it here. Maybe, just maybe you can use it like you did, but I really doubt it.

I hope that this is clear to you. :)

　We must write "in the practice test" because we use "the", which means that we are referring to not some random 200 problems, but the specific 200 problems from the practice test. :)

　"However" and "but" mean the same thing, but "however" should be used at the beginning of a sentence, or if used in the middle of a sentence, would be used with a semicolon and a comma (...; however,...). Note that when we use a semicolon in the middle of a sentence, the sentence that comes before it and the sentence after it will be sentences that can stand by themselves, for example, "Jane did keep her room clean; however, she did not keep her bathroom clean.

"But" should be used in the middle of a sentence. Using "however" in the middle of sentences and "but" in the beginning of sentences aren't really wrong, but they are not really right either, if you get what I mean. The rules of "however" and "but" are twisted a lot because the people who write dictionaries, the big name grammarians, and reputable writers all have different opinions, but it is safer to use "however" and "but" the way that I have described.

We are bringing up the doctoral thesis for the first time in this piece of writing, so we use "a/an". :)

　I think that writing "from today onwards" is more natural. We can also write "tarting from today". :)

　I think that this is more natural. :)