Hi Lalala, I believe at some point nearly everyone wonders about pruning lilacs. I watched a neighbor do a very heavy pruning of her beautiful lilacs and she lost nearly all the blooms for the next year. I do not have any lilac bushes in my gardens but I found an informative article from University of Vermont Extension that may be helpful to you.
Welcome! I’m Margaret Roach, a leading garden writer for 25 years—at ‘Martha Stewart Living,’ ‘Newsday,’ and in three books. I host a public-radio podcast; I also lecture, plus hold tours at my 2.3-acre Hudson Valley (NY) Zone 5B garden, and always say no to chemicals and yes to great plants.