In Spanish, you’ll see sin embargo or no obstante at the beginning of a sentence to signal however or nevertheless.
Sin embargo, hay otros factores más relevantes.
La asamblea resolvió el tema por unanimidad. No obstante, hay quienes dicen que algunos votaron bajo presión.
This works in English, too:
However, there are other, more relevant factors.
However, there are those who say some voted under pressure.
The meaning is clear—a shift in focus. But in English, however can move around. You can use however at any place in the sentence to emphasize different parts of the sentence, or even make a joke.
Using however at the beginning of a sentence generally signals that you are going to contradict your previous thought, or at least add a complication!
Rain was not predicted in the forecast. However, it poured. [You could also use the more formal nevertheless in this example. It means in spite of that or in contradiction to that.]
I ate the eggplant just to be polite. However, I was surprised to find I enjoyed it!
In the middle of the sentence:
We are horribly late to your dinner party. We did, however, bring you flowers. [This one would be light hearted, with the “however” signaling that flowers would somehow make up for missing the party].
At the very end of a sentence, however signals an afterthought or an alternative:
We just sold the last couch in that color. There is a matching loveseat, however.
Another time however appears is similar to no importa:
However we do this, it needs to be done. [No importa cómo hagamos esto, hay que hacerlo. Como sea que hagamos esto, es necesario hacerlo.]