How to read an antardasha
Mahadasha is the season, antardasha the day. Read them together.
A mahadasha can run for eighteen years. Nobody's life has one flavour for eighteen years, and that is what the antardasha is for — the mahadasha divided again, in the same fixed sequence, proportional to the same year counts.
The arithmetic
Inside a mahadasha of N years, each graha's antardasha lasts N × (that graha's years ÷ 120). So inside an 18-year Rahu mahadasha, the Guru antardasha runs 18 × 16 ÷ 120, which is 2.4 years. The sequence starts with the mahadasha lord itself and then continues in the standard order.
Reading the pair
Read them together, never separately. The mahadasha lord sets the theme; the antardasha lord decides how that theme shows up right now. A Rahu mahadasha with a Budh antardasha behaves like ambition expressed through talking and trading. The same Rahu with a Shani antardasha behaves like ambition meeting a wall.
Mahadasha is the season. Antardasha is the month. Pratyantar is the day.
The common mistake
People look up what a graha "means" and stop there. What matters is where that graha sits in your chart, which bhav it owns, and what it aspects. Two people in a Shani antardasha at the same time will not have the same year, because Shani is not in the same place in their charts.