You will most likely not be able to marry Kai until your third Summer.
Here are the requirements you must complete before you will be able to marry anyone:
Upgrade your house twice
Buy the Big Bed
Have a Red Heart with the guy
See all of his Heart Events
Rescue 60 Harvest Sprites
Since Kai is only around during Summer it takes awhile to get his Heart Color to Red.
The best thing to do is to buy a few bags of Pineapple Seeds before the start of Summer. Then on Summer first plant the Pineapple Seeds. Pineapples are Kai's favorite gift.
If you plant them as soon as Summer starts they should be grown in time for Kai's birthday. Which is Summer 22nd. Gifts are much more affective on the person's birthday. You'll gain a huge boost of affection if you give him a Pineapple on Summer 22nd. Any gift will work but you'll get the most is it is a Pineapple.
Once Summer is over and Kai leaves save the left over Pineapples for when he comes next year. Then as soon as he comes back you'll be able to give him his favorite gift.
During the rest of the year you can focus on getting the necessary amount of Harvest Sprites and other requirements like upgrading your house.
Three weeks
The pregnancy lasts for 60 days
21 days.
Around 60 days but if you forget your husbend will tell you the day before you have the baby
That depends on which Harvest Moon game you are playing. Most of the Harvest Moon games go on for as long as you play it though.
Nobody, your son in harvest moon DS will never grow past the teen age, get married, move away, die, or help with the farm work, no matter how long you play the game.
Keep your spouse happy and two seasons after your married you or your wife will get pregnant.
Well, when you get married, after about 6 months into the game your wife will get pregnet and it'll take about 7-10 months for the baby to get born on Harvest Moon DS. Right now, Celia is my wife and she is pregnet but I had Muffy as my wife on Harvest Moon, PlayStation 2 and we have a child. It takes a while.
Yes, you can still see them. As long as you have all the requirements for them to happen met.
It takes a while to get married. The thing that normally takes the longest is getting 60 Harvest Sprites, which is one of the marriage requirements. If you know what you're doing you could get married in less than two years. Unless you're trying to marry a Mineral Town character. Then it takes at least three years to marry Kai and usually four years to marry any of the other Mineral Town guys.
Two seasons after you are married you will get a pregnancy event. One season after that your child will be born. All you have to do is play long enough to get the scenes.
You must keep your spouse's love point at 60,000+ for 30 days. About a season after you have been married, your character will wake up feeling ill. After 60 more days, the baby will be born. Your spouse will also remind you a day before the baby is due.