"Raj is Julia's sister's son."
Translation:राज जूलिया की बहन का बेटा है।
The difference between की and का is that while the former is used when the object being talked about is feminine, the latter is used for masculine objects. However, it does not depend on the subject's gender. For example, consider the sentences राज का सेब (Raj's apple) vs राज की किताब (Raj's book). Since सेब is a masculine noun, का is used, while की is used for the feminine किताब.
Nothing. It is a way to clasify nouns in some languages. If a noun is masculine, it will use masculine adjectives or masculine definited articles or whatever. In english this doesn't happen because the nouns don't have specific articles or specific adjectives. For example, the word "cow" or "book" will use the universal definited article "the" in english, so you say "the cow", "the book", but in some languages (hindi, french, spanish, portuguese, italian...) have masculine/feminine words (there are other languages like russian, polish, greek, german... where they use masculine/feminine/neuter). So, in german (for instance) you have to say "Die Kuh" (the cow) and "Das Buch" (the book), using Die as feminine article, and Das as neuter article, because cow is a feminine noun in german, and book is a neuter noun. Or "La vaca" (the cow) and "El libro", in spanish, where La is the feminine definited article and El is the masculine one. In hindi, some words ending in -a are masculine, and some words ending in -i are feminine, but not all of them, so you have to memorize them by heart. In the case of the nouns सेब "apple" and किताब "book" in hindi, apple is masculine, and book is feminine. Not a particular reason for that, really.
How to cheat if you don't have an indian keyboard: copy paste the translation from this page! (I only do it as I have no alternative to go forward, no indian keyboard, and not available on the app. I know it's somehow against my objective of learning the language, please do not make any moral comments)
I included the hindi keyboard (Language preferences: add a language) and then searched online for the traditional hindi keyboard. It is very time consuming but it works. You can change keyboards easily by pressing the appropriate language on the inferior right side of your screen (for windows). Nevertheless it does seem indeed very time consuming and hard for beginners without a hindi keyboard...
For the benefit of others following this post, you do not need a physical Hindi keyboard. If you have a computer or an Android cell phone, you can install a software keyboard that will be on screen and free. https://www.androidpolice.com/2017/10/04/google-indic-keyboard-reaches-100-million-downloads-despite-eclipsed-gboard/
You can then use your mouse to select the Hindi characters or in the case of a cell phone you can switch to Hindi (character) input. Google Translate will also let you draw the characters with your finger on your cell phone screen. I have to say that this has helped me tremendously in my process of learning Hindi.
If you have an Android device you have to go to the Google Play Store and search for the Google indic keyboard app. Once you install this application on your cell phone, go into your Android settings and make this keyboard the new default keyboard. Don't worry the keyboard can do both English and Hindi and then when you go to enter Hindi on Duolingo you simply switch to Hindi and start typing the Hindi characters into the field. Duolingo doesn't have a built-in keyboard you have to install it on your device.