Writing rules for hiragana
In Japanese, there are short and long vowels, as well as double consonants. Japanese people are really sensitive to those sounds; as such, it is very important to pronounce the words correctly, otherwise you might say a totally different word.
You will see those long vowels and double consonants in writing as well.
Each vowel can be elongated.
To make a long あ, simply double the syllable by adding another あ.
おばさん (aunt)
おばあさん (grandmother)
To make a long い, simply double the syllable by adding another い.
おじさん (uncle)
おじいさん (grandfather)
To make a long う, simply double the syllable by adding another う.
すり (pickpocket)
すうり (mathematical principal)
To make a long え, double the syllable by adding a い.
えき (station)
えいき (energy, spirit)
てき (enemy)
ていき (commuter ticket)
Some exceptions will happen.
おねえさん (big siter)
To make a long お, double the syllable by adding a う.
も (also)
もう (already)
ほ (sail)
ほう (law)
Some exceptions will happen.
おおきい (big)
こおり (ice)
とおり (street)
おおさか (Osaka city)
Keep in mind that the う followed by the お sound makes an elongated お and thus, these words should not be pronounced 'mo-u' or 'ho-u', but 'mo-o' and 'ho-o'. However, the only instance when you will need to pronounce the う is with verbs in their dictionary form, such as the verb おもう (to think). It should be pronounced 'o-mo-u' and not 'o-mo-o' as it is a verb.
Be sure to clearly pronounce the elongated vowels, as Japanese people are very sensitive to these sounds and will make a difference between long and short vowels. As you can see in the examples, they are not the same words.
As for the double consonants, you will see them in words with a mini っ. The mini っ generally indicates that we need to make a small break in between consonants. It is like a pause you make between two syllables. Have a look at these words:
がっこう (school)
いっち (one family)
もっと (more)
しっと (jealousy)
There will be a small pause between 'ga' and 'kou', 'i' and 'chi', and so on. Again, you need to be mindful of this, because if you do not make a small pause, you could say a different word.
いき (breath)
いっき (riot)
はかい (destruction)
はっかい (8th floor)
べし (must, command)
べっし (contempt)
At first, it might be difficult for non-natives to see the mini っ, thus pronouncing a word the wrong way. You will see that the mini っ is smaller than the rest of the hiragana.
Finally, we have the moraic consonant ん. This hiragana works differently than the double consonants, as it will be pronounced.
こんにちは (hello)
ざんねん (disappointing)
This consonant will be pronounced the same amount of time as other hiragana.
せんせい (teacher)
てんぷら (tempura)
もんだい (problem)
Each syllable in Japanese have one mora. Moras are the beats in Japanese. It means that every syllable will be pronounced the same amount of time. もんだい will be pronounced も・ん・だ・い. One syllable equals one mora.
And this is it for the writing rules for the hiragana!
Hiragana are straightforward and do not require tons of rules.