Actually, (sentence)

Start a sentence with "Actually..." when you need to ask someone to change plans or do something different than they expected. Beginning with "Actually..." is polite and makes it seem like you're sorry for inconveniencing the listener.

Could you (do something)?

Ask someone to do something with this phrase.

"Could you ___?" is pretty neutral, so you can use it in a lot of different situations: with people who are higher status, lower status, friends, strangers, etc. 

I had a question about (something)

People sometimes ask questions in classes, speeches, and presentations this way:

I had a question about what you said earlier about drug testing regulations. Can you explain what you meant when you said...?

You ask this way in classes, speeches, and presentations because you can't interrupt the speaker. So you have to "save" your question until the right time, then explain to the speaker what topic your question is about.

the previous slide

When people give presentations, they sometimes use "slides". Slides are still images that you show, one after another, while you're giving a presentation.

Slides go in order. You can talk about different slides by explaining where they were in the order. The slide that you're looking at is "this slide" or "the current slide":

This slide shows how our manufacturing process works.

To descibe the one before the current slide, use "the last slide" or "the previous slide":

On the previous slide, you listed the number of unemployed workers in Europe. Where did that figure come from?

To describe a slide earlier than the previous one, use "an earlier slide" or "one of the previous slides".

Talk about slides after the current one with "the next slide" or "a later slide".

go back to (something)

When someone is teaching something, explaining something, or telling a story, you can use the phrase "go back to ___". This means to repeat or give more detail about part of the explanation or story.

For example:

Can you go back to what you were saying about Mike getting arrested? How did that happen?

I asked her to go back to the part about DNA replication, but she said she didn't have time to explain it again.