'to boldy go' is probably the world's most famous example of a 'split infinitive', which is when you put the adverb between 'to' and the verb. While it is not actually incorrect to do this, the adverb usually goes after the verb.

The position of the adverb that modifes a verb depends on the kind of verb you are using. Verbs expressing how often something is done must have the adverb before the verb: e.g., I always go the post office on a Tuesday. On the other hand, adverbs that modify the manner or way in which something is done go at the end of the sentence - not just after the verb, but after the object noun also I spoke to him politely. I listened to your lecture attentively. The position of the adverb that modifes a verb depends on the kind of verb you are using. Verbs expressing how often something is done must have the adverb before the verb: e.g., I always go the post office on a Tuesday. On the other hand, adverbs that modify the manner or way in which something is done go at the end of the sentence - not just after the verb, but after the object noun also I spoke to him politely. I listened to your lecture attentively.

Either. "To walk slowly" is correct. "To boldly go where no one has gone before" is also correct. I believe it's a little more common to place the adverb after the verb in conversational speech, though.

Please let me know if you can't get the book. In that case, I will find a way to translate the part of adverb for you.

In case you're in Thailand, buy a book with the title "Lak Pasa Thai" written by Praya Uppakit Sillapasaan. In that book, the writer provides clear details about the five types of adverb with their positions in the statement and meaning. You might be able to get less confused by studying the explanation and the examples.