"The bird flies."
Translation:पक्षी उड़ता है।
Unfortunately, there isn't one that is foolproof. Like in most other Indo-European languages with grammatical gender, the gender assigned to a word is based on a combination of many different factors such as word morphology, word-ending, etymology, meaning etc. You will get a somewhat intuitive hang of it only through familiarity with the language. Until then, it might be a good idea to learn any new word you come across along with its gender. That is, instead of learning कुत्ता='dog', बिल्ली='cat', you learn मेरा कुत्ता='my dog', मेरी बिल्ली='my cat'.
There are certain rules of thumb based on word ending (words ending with ा are more likely to be male, those ending in ी are more likely to be female etc) or the meaning of the word (country names are usually male, rivers are usually female etc) which are useful to make guesses but there are a lot of exceptions to each of these 'rules'.
For example, पक्षी is an exception to the ' ी -> feminine' rule because it's a direct borrowing from Sanskrit and brings over its masculine gender from Sanskrit. Then there is चिड़िया which also means bird (specifically a small bird like a sparrow) and being a native Hindi word should follow the ' ा ->masculine' rule. But it doesn't and is feminine (because of its meaning in this case).
है marks the sentence as being in the present tense. So, पक्षी उड़ता है is 'The bird flies' as opposed to 'पक्षी उड़ता था' which is in the past tense ('The bird used to fly').
Almost all present tense sentences in Hindi will have the है (or one of its other forms हैं, हूँ and हो). It can only be omitted in negative sentences. One other instance when present tense sentences don't have a है is when you have चाहिए.