I understand that "Ik hou van je" means I love you. Now, je, I think, is informal. It may be that there is no way to say this in a formal way.
"I hou niet van moderne art", I think, means I do not like/fancy modern art.
So in the first example we have love and in the second we have like.
Liefde means love but in the above sentence there is no verb simmilar to this noun.
Could you please explain all of this?
Tack sa mycket för det frâge. Does that sound right? I have learned a little Swedish because of some work in Sweden.
Your question is not entirely clear to me. I can see multiple things that you might want to know about.
First of all "Ik hou van je" does indeed mean "I love you". In written language, you should write "Ik houd van je" because the verb "houden" has as stem "houd". I am not sure but it might have an etyomological basis in "to keep" or "to hold" because "Ik houd bijen" means "I keep bees" and "Ik houd vast" means "I am holding on". You can see that "houd" and "hold" are rather similar, because our languages share the same ancestry.
We make a distinction between "je" and "u" (or "U" more formally like the German "Sie") to show the distinction between the informal form and the formal one. A child could say to one of their parents "Ik houd van u" though that is not so common these days any more. Children are raised to use the informal form with their parents to get closer to each other. Personally, I think that is nonsense; I raise my children to use the formal way so that they can cope with formal situations in the world when they are older. It does not create any distance as they don't see the difference. They just use another word, in their minds. But that's enough about me.
When you say "Ik houd niet van moderne kunst" then you just replace X with Y so "Ik houd van X" can be filled in with anything. "Ik houd niet van Surströmming". You know what that means now.
In Dutch there is little difference between "I love ..." or "I like ...". In both situations you can use "Ik houd van ...". But if you want to make sure you don't overstate it, you can say "Ik vind ... leuk" which can only be translated to "I like ...".
In your explanation you talk about "liefde" and "love". Please note that there is a difference between the verb "to love" and the noun "love". The first translates to "houden (van)" (you have to understand grammar of both English and Dutch to translate an entire sentence correctly), the latter to "liefde". To make it more complicated "Ik ben verliefd op jou" does use a verb "verliefd zijn (op)" but it translates to "I am in love with you" where we use the noun "love" (because the verb that we are using in these sentences is "to be" which in Dutch is "zijn".
Please et me know if this answers your questions.