The consonant 'i' and syllables

Hello!
I was going through the Latin for beginner’s by D’ooge. It says that before a vowel the letter ‘i’ is a consonant in the same syllable and and in all other cases, it is a vowel. However, when dividing the word “viās” into the syllables, how do I find that ‘i’ here is a vowel and or a consonant? Because I won’t know how many syllables it contains before I find how many vowels are there. Please explain. Note: The letter ‘i’ is accented here.

Another one. the word “cōnsīderā” contains four syllables “cōn-sī-de-rā”. Why can’t it be like “cōns-ī-de-rā”? Am i missing any rule? Also is the letter ‘i’ before a diphthong also a consonant in the same syllable? And how do I identify it as a consonant? Same problem as above.

I’m not the greatest expert in these matters, but since nobody else has said anything, I’ll tell you what I know (or think I know) about syllables in Latin:

  1. In Latin, every syllable contains at least a vowel sound (V): The most basic kind of syllable contains nothing but a single vowel. Its structure can therefore be described as V (e.g. the prepositions “e” and “a”).

  2. If you add a consonant before the vowel, the structure of the syllable can be described as CV (e.g. “de”, “mi”).

  3. If you add another consonant at the end, the structure is CVC (e.g. “non”, “fit”).

  4. As a general rule, every syllable in Latin wants (so to speak) to start with a consonant, if one is available. This is why we have “a-mo” (V-CV), not “am-o” (VC-V), “de-ge-re” (CV-CV-CV), not “deg-er-e” (CVC-VC-V).

  5. If two consonants are available between two vowels, they are usually divided up between the first an the second syllable, e.g. “dan-tur” (CVC-CVC), “dic-ta” (CVC-CV). There are some exceptions though, which can be important in poetry.

With regard to the word “considera”: It cannot be divided as “cons-i-de-ra”, since this would leave the “i” alone and unhappy (not the technical term). As I said above, every vowel wants to have a consonant at the start of its syllable if one is available. This means it has to be “con-si-de-ra”.

Regarding “vias”: If the “i” were a consonant, the word would be quite unpronounceable, wouldn’t it? At least in Latin you can’t have a word with a structure like that.

Thank you very much. Your help has been fruitful.