In Google I can type in, "What's the weather like in Vancouver this weekend?" and I will receive the answer. You need to dumb down your inquiry to short, important pieces of information for database searching.
For example, if I am doing research on patient care in nursing homes and the long-term affects of poor care on the residents within a database (perhaps CINAHL) I do not want to type in that ENTIRE sentence. Really I am looking for the meat of my sandwich: Nursing Homes / Patient Care / Elderly / Resident Affect
Once you have the meat, you can then start to put together a search strategy; (see image below). You should consider synonyms as well. Perhaps other authors refer to dogs as canines in their article or specifically the breed German Shepard. You want to make sure you get ALL of the relevant possibilities for your research.
My search would look something like this:
Try these techniques for limiting your results:
Try these techniques for increasing your results: