A chicken in the yard 庭にはにわとり

As long as you're not trying to accentuate the fact that it's your house, it's definetly not required. It should be correct even without it.

小さな is an adjective used to say something directly about an object. Example: 「これは小さな池です」meaning 'This is a small pond.' 小さい is an adjective used to say something about an object in general. Example: 「この池は小さいです」meaning 'This pond is small.'

Learn the hiragana and katakana. Work on the kanji. Practice 1000 basic vocabulary. Find a friend who speaks to practice with.

I think it is quite clear that the duolingo Japanese course cannot teach you the language on its own (to the extent that the other duolingo courses can, that is). There are many other good sources of learning, though, which can be used to supplement that which duolingo cannot teach. I recommend the following apps:

Obenkyo, for an offline grammar guide, as well as practice writing hiragana, katakana, and kanji (all of them). It has character recognition as well as multiple choice. Link:

https://play.google.com/store/apps/details?id=com.Obenkyo&hl=en

HiNative, for asking questions and correcting your writing in the language. Also for the satisfaction of answering other people's questions!

https://play.google.com/store/apps/details?id=com.lang8.hinative

Counting in Japanese, for practice with counters (you're going to need it!).

https://play.google.com/store/apps/details?id=com.japanesecounting

And some others, like drops, memrise, Japanese verb conjugation, NIK日本語, and tinycards.

I know I'm a bit late, but hopefully other people will find this list helpful!

I'm curious: how do you find the time to study and keep track of over 30 languages, many of which you have reached impressive levels on. I struggle with just 2!

This may require a bit of a lot of explanation. If you're feeling like doing a lot of reading, here's a nice link to answer your question in depth:

http://www.guidetojapanese.org/learn/grammar/particlesintro

For a brief explanation, は plainly states something as the subject, while が identifies one thing out of a group as the subject. To plagiarize the link I just sent:

私は学生。 As for me, (I am) student.

私が学生。 I (am) the one (that is) student.

The former would be used if someone asks "what do you do right now?" and the later would be used if someone asks "who among you is the student?"

In the case of this sentence, が is used to say that a pond is the thing in my yard. I've never seen anyone say はあります because that wouldn't make sense.

Hope that helps!

に refers to the location that remains in a static state under the verb. で refers to the location where action verbs take place like swimming or climbing because they change the state of the location (like how swimming or climbing is happening in the location). Here, the verb ある describes the existence of something and thus no action is performed with regards to the yard. The yard remains static, so に is used.

The particle that is used, is actually 「には」(please correct me if I'm wrong) As to the question 'why?': I don't really know. I also tried to use で in this sentence. は (as the second part of this particle) is normally used to emphasize the subject of the sentence.

The explanation of ObitoSigma is very confusing and I think it is even wrong. Because in other sentences に is used for something or someone moving in or into a location and で is used for static objects in a place.