Originally posted by pepperlime on The Sims 3 Forums. Posting here as I thought it was pretty helpful and will hopefully help others.

It took me a while to figure out how to talk to other sims and get what I wanted. Honestly the dialogue/relationship system feels oooold old, like ts1 pre-Hot Date expansion. Sims don't talk to one another over meals (wtf?), there's just one relationship bar, and you can get someone to marry you in like a day.

First I get the "Observant" lifetime reward asap, because discovering traits of other sims is a big relationship boost right off the bat (if they're the same as yours). With this reward, you know 3 traits right as you introduce yourself. But sometimes getting the next 2 takes forever.

Think of the opinion of the other sim as something you build up (over and over) to let you do what you want. If it says "... thinks you are being very social" you can do the most advanced friendly interactions, like asking them to move in. If it says "... thinks you are being extremely irresistible" you can do the most advanced romantic actions, like woohoo or ask to marry. Often, when you choose an advanced option, their opinion of you resets to "... thinks you are okay" (so you used up whatever opinion-power you had built up). The exception to this is "first kiss", which the game doesn't consider an advanced romantic option (apparently). Or if you chain 5 romantic options and then a friendly option, it will reset and your progress in the romantic direction will be lost.

Yeah, moving in is more of a rooommate/friendly thing, even if they're your boyfriend/girlfriend. It's an advanced option of the friendly category.

Once the bar is 1/3 green, you can flirt (I play this safe). After that it's only a short time until they marry/woohoo, assuming they don't run away randomly from the conversation. If they're over your house, inviting them in keeps them from doing this as often.

If you mess up and the other sim's opinion becomes "... thinks you're awkward" or "creepy", apologizing can reset it back to "... thinks you're okay".

Dancing together makes your relationship bar go up slowly but steadily.

Talking about a trait or job two sims have in common is the only way I've found to get a double "++" positive interaction.

Chatting on the phone makes your relationship go up, but only in tiny tiny amounts. It's much better to invite a sim over, go to their house, or go to the park and try to accidentally run into them. If they don't want to come over when you invite them, try again a few times. Even if your relationship bar is only a tiny bit green, they might say ok after a few tries.

Relaxing on the bed gives you the option to woohoo/try for baby right away, but if your relationship bar isn't at full, then you should try chatting/cuddling first. Otherwise the other sim will reject your advances and then think you're awkward or creepy.

Added by beaynidSim

To get a couple engaged, you need to build up the relationship and interactions to 'go steady'(it will make them a couple), then after that they can 'propose marriage' and become engaged, and finally its either throwing a wedding party, or get married privately