Merhabalar! I know that this question is probably too open-ended, but I will try anyway. Is there a guideline of some sort to tell when you should say "-de/-da" and when you should say "içinde"?

For instance:

Kedi bahçede uyuyor.

or

Kedi bahçenin içinde uyuyor.

That may not be a good example, but you get the idea of what I am trying to ask. Teşekkürler! 

i think there is no differnce between them

but 

kedi bahçenin içinde uyuyor --> certain

kedi bahçe içinde uyuyor --> uncertain

Good question.

Unlike English, the different prepositions in,on and at are not used much in Turkish. Instead, "-de" covers them all. Examples:

-Evde (at home) nobody says evin içinde

-Bahçede (in the garden)  

-Okulda (in/at the school)

-Çantada (in the bag) you can also say çantanın içinde

-Televizyonda (on tv)

-Tatilde (on vacation)  

-Yatakta (in bed)   nobody says yatağın içinde

-Kağıtta (on the paper)

-Masada (on the table)  or masanın üstünde

As you can see, you don´t need to use different prepositions unless you want to emphasize the exact position of something/someone. For example, when you say "Okuldayım", it means you can be anywhere on school premises. (Like I´m at school in English, I guess). So, let´s say your friend is trying to find you and you are giving him clear directions, in that case you might want to say "Okulun içindeyim" (I´m inside the school building, or whatever).

Coming back to your original question, "Kedi bahçenin içinde uyuyor." would sound unnatural to me (although gramatically fine) unless there is such a context. When I think about it, "bahçede" doesn´t lead to any ambiguity in the meaning anyway, right? (you can´t say at the garden or on the garden). Therefore, I think you wouldn´t need "içindeyim" in any case.

That is a very clear explanation, gugukkusu. I thought there must be some secret formula that I had missed, because somehow whenever I come up against a sentence like this at Duolingo, I manage to give the wrong answer. They often seem to want the "içinde/üzerinde" construction. Using -de/-da just seems so much quicker and easier. So I feel much less stressed now.  Çok teşekkür ederim!

içinde = ´inside the´      (at the inside of the)

-de/da = ´in´               (at/on)

"içinde", "üstünde" are often dropped when there is no ambiguity or when no special emphasis is required. We can say "masanın üstünde" (on top of the table) but "masada" (on the table) would be sufficient because, if it had been somewhere else, e.g. altında (under) or yanında (beside), this would have been stated.

Since "üstünde" has the meaning of "on top of", for hangings, we often use "duvarda" (on the wall.)

Now, let´s imagine a medieval castle under siege, soldiers defending the castle are " duvarın üstünde" (on top of the wall) but not, as they are not hangings, "duvarda" (on the wall.)

We often say "yatakta" (in bed/on the bed) but, as you may guess, it is not always clear whether or not you are lying under the covers or lying on top of it. When an emphasis is required that you are under the covers, saying "yatağın içindeyim" instead of "yatakta" never sounds weird. You can tell your little kid who´s about to fall asleep on top of the covers; "oda soğuk, yatağın içine gir" (it is cold in the room, get inside the bed)

and so on...  

Yes, I was just trying to make it short and simple

mine was long and puzzled? (

puzzled?    Complex is a better word to use

For foreigners it´s a lot to grasp. I know, for me, I like to have a good explanation of the grammar but also something short I can actually remember fast enough when I need it. Now Trip has both. Just trying to be helpful guys