This is a dialectal/accent thing. The Tokyo dialect is the national standard; earlier in Japanese history, the Kyoto dialect was standard. There is at least one dialect per prefecture in Japan, and some of them (e.g. Kansai) pronounce the "u" part of す more clearly than in the Tokyo dialect.
This is called a ten-ten, and it marks impure sounds. For て for example, adding a tenten (で) will change the sound from te, to de. The change in sound depends on which sound (ka, ga, ma, etc.) you do. For example, any ka sound with tenten turns it into ga, sa is za, and so on.
This is, well it's also a tenten, but usually the word dakuten is used. It makes the consonant voiced. (voiced means that you use your vocal chords while pronouncing it, think of the difference between s and z, they are the same except for the vibrations in your throat, some other unvoiced-voiced pairs are: t d, p b, k g, s z, sh zh (j in japanese), ch j, ts dz)
I saw someone answer already, but i wanted to reccomend a YouTube video titled "Learn All Hiragana in 1 Hour" by the channel JapanesePod101. I learned all my Hiragana there before jumping into DuoLingo and id say it helped tremendously. Take it slow! its an hour long video but it took me 3 weeks to get all the way through it. Go at your own pace!
Yep! Duo will start introducing kanji right after the hiragana skills in Intro1 :)
I will say though that kanji is a big enough topic to get a course all in itself so Duo isn't really equipped to teach it in depth. There are multiple apps and books and websites that focus exclusively on teaching kanji. Kanji Garden, Kanji Tree and WaniKani seem to be the top recommended ones by learners here. You can also learn them by using a good dictionary like Jisho.org which provides stroke order, readings, examples, etc.
It seems as though pronouncing the full letter should rarely if ever be "wrong." The issue however is with whether or not native Japanese speakers tend to fully pronounce certain sounds fully or "skip" over them. In English, there are countless cases where some people "skip" syllables in a word even when they shouldn't "really" be skipped, simply because it is what they are used to hearing and/or speaking. As somebody who is attempting to learn Japanese however, it is frustrating to see a lot of sounds on this site not pronounced when we're attempting to learn what sounds each character is supposed to make.
If you're English-speaking, you can think of it in a similar way as "I am" versus "I'm". Skipping vowels for speed and efficiency is a quite common phenomenon in languages. Some languages evolved with time in such a way that they bundle several consonant sounds next to each other (by dropping vowels that once have been in between them). See, for example, the English word "strength", which is one syllable nowadays.
That's voiceless vowels for you. :) In Japanese, a vowel occurring between two voiceless consonants tends to be devoiced. The 'u' in "suki" is sandwiched between the voiceless consonants: /s/ (voiceless alveolar fricative) and /k/ (voiceless velar stop) and hence ends up being voiceless. The interesting this is that voiceless vowels aren't an alien thing at all. They are precisely what occur in place of normal vowels when we whisper. So to pronounce "suki" with a devoiced 'u', all you need to do is say the "suk" part as if you were whispering. :)
The meaning of Japanese sentences very strongly depend on the context. You can really yell to your boy/girlfriend "(だい)すき！" to express "I like you (very much)!" even this sentence has no subjects or objects. If your friend asks you "Do you like it?" he may uses "すきですか" (か=question marker), and you can even just simply answer "すきです。" to mean "Yes, I like it."
Almost, you're missing a few pieces.
First you need some particles. These are pieces of grammar used to mark what each function each word plays in a sentence. Japanese doesn't rely heavily on word order to determine function like languages like English does.
私は寿司が - watashi wa sushi ga ,
は "wa" marks "I" as the topic, the overarching thing we are having a conversation about.
が "ga" marks "Sushi" as the subject, the more specific thing we are relating to ourself.
好き・すき・suki is called a "na" adjective. This is an adjective that functions similar to a noun and it cannot be used to end a sentence. You need to add the copula です desu (functionally similar to the verb "to be")
The full sentence would be
私は寿司が好きです・わたしはすしがすきです・watashi wa sushi ga suki desu
broken into its grammatical components it's like saying (On the topic of me) (Sushi is the thing) (That is likeable
Last note, pronouns are rarely used in Japanese speech. If they can already be implied from context who you are speaking about they can be dropped entirely.
Since the listener can probably assume you're making a comment about yourself a simple 寿司が好きです・"sushi ga suki desu" is enough information to be completely understood. :)
好き is actually an adjective that means "likeable". If something is likeable (好きです), that means you (or someone else) likes it. (あの人が好きです = (someone) likes that person) You can also use it as an adjective: 好きな人 = a person whom (someone) likes.
The thing that likes is given by the topic. 私は日本が好きです。 = I like Japan. Of course, you can leave out anything that's obvious.
In Japanese it is actually used as an adjective, similar to "likeable, favorable"
The translation used to say "like" without the s; I suspect the s was added to help clear confusion between the verb/adjective "X likes Y" (correct) and the comparison "X is like Y" (incorrect)
好く・すく is the verb form but it is pretty much never used, it's become obsolete. The noun form of the verb 好き・すき used as an adjective (the one taught here) is the most common way to say you like something.
There is also 好む・このむ which rather than 'like' means something closer to "To prefer"
Thanks for the compliment. However, I will repeat myself in English if something is not clear enough. If a u or i is pronounced next to a sonorant consonant or at the beginning of the word, it's not dropped. If that vowel is pronounced between two mute consonants or at the end, then it COULD be dropped. But as it's only a possibility, then you still have to be guided by your guts.
You can always pronounce it and you won't be wrong. Choosing to pronounce it is more personal preference than a rule. When hearing other people omit the "u" vowel you can always tell because there is no way to write the syllable without a vowel in the Japanese writing system.