1.首付
(1) down可以表示“当场支付，首付”
英语解释为paid to someone immediately。
(2) pay an amount of money down “支付...首付”
例
How about zero down and 10,000 a month for a long time?
0元首付，长期1万元月付怎么样？
He bought a car for 40,000 down and 2,000 a month for three years.
他买的这辆车需要4万首付款，余额月供2千，三年内付清。
2.首付
(1) down payment “（分期付款购物的）首付款，首期付款”
英语解释为a payment you make when you buy something that is only part of the full price, with the rest to be paid later。
(2) make a down payment of an amount of money “支付首付......”
(3) make a down payment on sth. “支付......的首付”
例
That down payment was for a house. You bought a car.
那是买房用的首付款，却被你用来买车。
He loaned us the money for the down payment.
他借给我们首付的钱。《摩登家庭》
3.房贷
mortgage [ˈmɔːrɡɪdʒ] “房屋抵押*款贷**，房贷”
例
They've taken out a 20-year mortgage.
他办理了一笔20年的房贷。
We have a mortgage. We have a child to support.
我们要还房贷，还要养个孩子。