user27480: Is there a website where you can play against real people with an opening already installed by default? For example, I want to play against the English opening, but my opponents rarely play it, so I want to practice somewhere.

user15223: There are theme tournaments on ICCF. Example for the Alapin variation of the Sicilian. However, it is correspondence chess, which might not be your cup of tea.

user27298: If you want, Chess.com allows you to play from a custom opening position against a computer, which you can choose your level. If you need help with it, go here. You can also send a game to a friend.
https://support.chess.com/article/684-how-can-i-play-the-computer-from-a-custom-position
Lichess does a similar thing. Create your personal board, and press continue from here to play with a computer or a friend.

user15224: Lichess also has several thematic tournaments throughout the week.
As an example take a look at this thematic Italian game arena tournament:

When you open the tournament, the opening is mentioned in the upper left corner if there is one:

It is a link so you can click it to see the mentioned opening.
Alternatively you can create your own tournament with a opening of your choice. For that just go to the create tournament area and set the starting position:

In that box you have to place a FEN string. For the English opening considering only 1 C4, you can use the following fen:
rnbqkbnr/pppppppp/8/8/2P5/8/PP1PPPPP/RNBQKBNR b KQkq - 0 1
Regardless you can create any fen of your choice with the lichess analysis board tool or the opening explorer

27480: <p>Is there a website where you can play against real people with an opening already installed by default? For example, I want to play against the English opening, but my opponents rarely play it, so I want to practice somewhere.</p>
 15223: <p>There are theme tournaments on ICCF. <a href="https://www.iccf.com/EventAnnouncement.aspx?id=87920" rel="nofollow noreferrer">Example</a> for the Alapin variation of the Sicilian. However, it is correspondence chess, which might not be your cup of tea.</p>
 27298: <p>If you want, Chess.com allows you to play from a custom opening position against a computer, which you can choose your level. If you need help with it, go here. You can also send a game to a friend.</p>
<p><a href="https://support.chess.com/article/684-how-can-i-play-the-computer-from-a-custom-position" rel="nofollow noreferrer">https://support.chess.com/article/684-how-can-i-play-the-computer-from-a-custom-position</a></p>
<p>Lichess does a similar thing. Create your personal board, and press continue from here to play with a computer or a friend.</p>
 15224: <p>Lichess also has several thematic tournaments throughout the week.
As an example take a look at this thematic Italian game arena tournament:</p>
<p><a href="https://i.stack.imgur.com/RSn4n.png" rel="noreferrer"><img src="https://i.stack.imgur.com/RSn4n.png" alt="enter image description here" /></a></p>
<p>When you open the tournament, the opening is mentioned in the upper left corner if there is one:</p>
<p><a href="https://i.stack.imgur.com/84U5o.png" rel="noreferrer"><img src="https://i.stack.imgur.com/84U5o.png" alt="enter image description here" /></a></p>
<p>It is a link so you can click it to see the mentioned opening.</p>
<p>Alternatively you can create your own tournament with a opening of your choice. For that just go to the create tournament area and set the starting position:</p>
<p><a href="https://i.stack.imgur.com/Wv2mu.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Wv2mu.png" alt="enter image description here" /></a></p>
<p>In that box you have to place a FEN string. For the English opening considering only <code>1 C4</code>, you can use the following fen:</p>
<p><code>rnbqkbnr/pppppppp/8/8/2P5/8/PP1PPPPP/RNBQKBNR b KQkq - 0 1</code></p>
<p>Regardless you can create any fen of your choice with the lichess <a href="https://lichess.org/analysis" rel="noreferrer">analysis board tool</a> or the <a href="https://lichess.org/analysis#explorer" rel="noreferrer">opening explorer</a></p>
