The combination of all 3 writing systems in a sentence makes it easier for you to separate words. With Hiragana being mainly grammar structures and some very easy and common words, most of the time you can identify Kanji and katakana as words, and have the hiragana with grammatical meaning separate the strings of kanji, as if the hiragana itself was the spacer. I don’t know if it made much sense. Here’s an easy example.
Kotoshi, kuruma de madoriddo he ikimashita.
This would literally translate to:
This year, by car, to madrid, (I) went
However, you shouldn’t read it like that. I really don’t know how to teach how to read it, it’s something that comes with time, but it’s like you keep reading and storing information, and then, when the sentence ends, everything makes sense in your head. This is one of the reasons why Japanese is so difficult to translate to English.
We see two kanjis at first, then a coma. This provides spacing. In this case, when you state a temporal information (“this year” in this case) you don’t need to add any hiragana to declare its function, although you could as far as I know.
Then we have another kanji: the one for car. After this kanji, we have the hiragana which is pronounced “de”, which means that the car is an instrument you used. It also helps separating it from the next piece of information.
Following is the katakana string that means Madrid (the city of Madrid). Next to Madrid, is the hiragana pronounced “he”, although in this situation it is pronounced just “e”. It means that Madrid is the place that there’s gonna be movement towards, it’s almost telling you that something is going there. Once again, the hiragana separates the katakana string from the next kanji, which would be the verb.
Finally, we have the kanji for “to go”, followed by a string of hiragana that is basically the verb conjugation in formal, past and affirmative.
I might have explained too much grammar, but that’s not the point, the bottom line is the following:
As you can see, while hiragana are the gramatical core of the language, they also do this function of separating words. Still, you can find some very fucked up sentences with amazingly long kanji strings that you have to decrypt, but those will most likely be used only to represent complex vocabulary.
Translators are shit when it comes down to Japanese. They’ll only somehow work if you input info in kanji, and for that, you should use a kanji dictionary to check that all the kanjis you are using are the ones intended and you don’t end up mixing up god and hair, which have the same pronunciation but different kanji.
Recognizing radicals is something that really comes with experience, but is one of the easier things after a bit. One of the methods to learn kanji is to learn the simple kanji that are also used as radicals before the full kanji with many radicals, even if the basic one aren’t used. An example would be this kanji 曜. It looks horrible at first sight, but if first you’ve learned that the part of the left is just like 日 (sun, day…) the two things on top are just like the katakana ヨ and the thing at the bottom, ⾫, is a small bird, when you come across this kanji you don’t see it as an abomination, but rather like a sum of things that you’ve already learned. Of course, 日 and ヨ are one of the first things you learn, but ⾫ on the other hand is rarely used outside its radical form, so you would only learn from this if you follow the method I described at the beginning of the paragraph. Otherwise, you’ll just learn over time that this part of the kanji is used in other kanji and, therefore, it is a radical.
Well, this is true for pretty much everything. If you want to learn something to a certain level of expertise, you’re gonna need to commit strongly at some point. This is especially true for kanji though, as you need to burn them into your name if you want to remember them during the rest of your life. This means drawing them several times, using them in sentences or reading them often.
Nonetheless, don’t feel overwhelmed by the initial shock of seeing text walls of unreadable information. I’m sure that if you follow along that book you’ll get the basics just right. However, it wouldn’t hurt to have someone expert in the language to talk to since people normally get things slightly wrong (nothing catastrophic though) when learning by themselves.