Use やっと when you want to say 'finally' after a period of waiting or effort. It can also mean 'barely' or 'narrowly' when the outcome was achieved but with great difficulty. For example, after a long train ride, you might say やっと着いた (finally arrived). If you finished a difficult assignment just in time, やっと終わった (barely finished) works too.

In terms of position, やっと comes right before the verb or adjective it modifies. It works with past tense actions, present states, and even future intentions. The nuance is always that the result was hard-earned—whether by patience or by struggle.

Note that やっと carries a sense of relief. It is different from ついに, which is more neutral and often used for significant events. やっと is more personal and emotional.