English grammar - correctly using FOR, SINCE, AGO

Here is a quick guide to using FOR, SINCE and AGO

FOR

For talks about a length of time (an amount of time) 10 minutes, 9 weeks, 25 years etc.

"I've known Dylan for 15 years."

15 years before ---------15 years----------- now

"Steven has been trying to catch a taxi for 30 minutes."

30 minutes before -------30 minutes--------- now

"When I was in University I lived in Toronto for 4 years."

2003---------------4 years-------------2007

日本語の例文

Steam for 10 minutes. 

10分間蒸す

I slept there for about 10 minutes.  

私はそこで10分ほど眠った。

SINCE

Since talks about a point of time in the past until now.

"I have known Dylan since 2009."

2009 --------------------> now

"Steven has been trying to catch a taxi since 6:30."

6:30pm ------------------> now

日本語の例文

I have studied in Paris since 2011 October.

2011年の10月からパリで勉強している。

AGO

Ago means before now.

"I ordered the pizza 25 minutes ago, it should be here soon."

= 25 minutes before

A: "When was your last dentist visit?"

B: "It was 7 months ago. I think it's time for another check-up!"