i'd like to visit Beijing tian'anmen, forbidden city, summer place & temple of heaven in the same day.

Hi, for the places that you are going to visit, I recommend this route:
Summer Palace - Tiananmen (Square) - Forbidden City - Temple of Heaven

The exit A of Beigongmen Metro Station of subway line 4 is closest to the North Gate of Summer Palace. After visiting that, you can use line 4 to Xidan, where transfer to subway line 1 to Tiananmen East. Getting out from exit A and you can walk to the square for a visit. And the Forbidden City is within walking distance from the square.

After that, walk to Tiananmen East Station to take suwbay line 1 to Dongdan and transfer to line 5 to Tiantangong Station. Get out from exit A1 and u can easily walk to Temple of Heaven.