When I listen to "watashi wa", for example, sometimes I have the impression that the 'w' sounds like 'v' as in "vial" or "violence" in English (Similar to 'w' in the German Language). Nevertheless, I also have the impression of listening 'w' as in "wonderful" sometimes, so I'm confused. Do the Japanese use both sounds? How 'w' is pronounced?
There is no "w", except in "wa", so you only have to learn that. And it helps to see a Japanese MC on tv with a microphone and a constant toothy grin, while talking furiously. This is an important key, because it demonstrates that Japanese can be spoken without ever moving the lips sideways. So the beginning of the 'wa' syllable is basically done by opening your lips like a letterbox, with purely vertical movement.
In loanwords, the combinations ウィ, ウェ, and ウォ also occur, but again these can be made with the microphone and grin uninterrupted, simply moving the lips vertically. But what cannot occur is ウゥ: not only does it not make sense, since you would be replacing the end of a vowel with itself, but typically Japanese speakers find it essentially impossible to pronounce (e.g.) "Woolworths", which demands that the "w" represents a sideways opening of the lips (twice!)
Japanese has two semivowels, /y/ and /w/.
The semivowel /y/ is pronounced like the vowel /i/.
The semivowel /w/ is pronounced like the vowel /u/.
や, try saying
いあ /ia/, but focus on transitioning smoothly from one vowel to the other. You'll end up with a
や /ya/ sound. Likewise, to say
わ, start by saying
うあ /ua/, but go smoothly from the first vowel to the second. You'll end up with a
わ /wa/ sound.
This is why Japanese has never had /wu/ or /yi/ sequences, even in Old Japanese. If you start with an
う sound and end with an
う sound, you just get /u/, not /wu/. Likewise for /i/, not /yi/.
Japanese /w/ is not pronounced [v].
To pronounce Japanese /w/ properly, you'll have to learn how to pronounce Japanese /u/ properly.
In IPA, the Japanese vowel /u/ is usually transcribed [ɯ], which is the symbol for a high back unrounded vowel, but it's not exactly the same [ɯ] sound you'll find in some other languages. Although in rapid speech it may be unrounded, in careful speech the lips are "compressed", which is a little different from what is usually called "rounded" or "unrounded":
In compression [ . . . ] the jaw closes and brings the lips together vertically so that the side portions are in contact, but there's no conspicuous protrusion. Figure 3-3 illustrates with a Japanese /u/. The clear lip compression shown in Figure 3-3 is characteristic of careful pronunciation (§2.12) in Japanese.8 In connected speech at normal conversational tempos, compression is generally weaker, and often totally absent. As a result, Japanese /u/ is commonly described as unrounded.
(The Sounds of Japanese, Vance 2008, p54-55)
And here's the picture from the book:
The Japanese /w/ is the semivowel version of the same sound. The IPA symbol for this is [ɰ] (and again, this may be a little different from [ɰ] in other languages because of the lip compression, at least in careful speech).
Once you can pronounce the Japanese [ɯ] properly, try making the same shape with your mouth, but transition smoothly to the following vowel [ɑ]. You should come up with a proper
わ /wa/ sound.
A note about phonotactics
Although in general in Modern Japanese /w/ only appears before /a/, you may hear a non-phonemic [ɰ] inserted in certain contexts before /o/, and in recent loans (外来語), you may come across the sequences /wi/ /we/ and /wo/ (spelled
ウォ in kana). Speakers may not pronounce these consistently today, but the sequences do seem to be re-entering the language through loanwords, and you may hear them from time to time. You may also hear them in colloquial pronunciations like
怖ぇ /koweʜ/, although other speakers may simply drop the /w/ as expected (resulting in /koeʜ/).
So it's not entirely true that [ɰ] is limited to