I can understand but I can’t speak.

This is an extreme version of the previous question. The first thing we need to do is to clarify what you mean by “I can’t speak”. Do you mean you can’t come up with any words? Do you mean that you hesitate when you try to speak? Does it mean that the language doesn’t flow out naturally without any effort? All these are very different things, and many of them have been answered in the previous question: I understand a lot more than I can say. If you really are in one of the more extreme cases, meaning that you can understand almost everything but cannot put together more than 2 words, it’s usually due to one of these two reasons:\n\nYou can already speak a similar language to the one you are learning. For example, you are learning Spanish as a speaker of Portuguese, French or Italian. In these cases, because so much of the vocabulary and the grammar is so similar to your language, you can guess what most things mean, even if you haven’t actually learned the Spanish version of those words yet. Evidently, it’s a lot easier to guess what something means than to guess how to say a word that you haven’t learned yet. In this case, you could be comparing your small active vocabulary with all the vocabulary that you can guess the meaning of. That difference is indeed extreme. You need to realize that just because you understand a lot it doesn’t mean that you have learned much Spanish yet. You need to keep getting input and actually acquire the Spanish words like everyone else. You’ll still be learning faster than anyone that doesn’t have this advantage. If you are in this situation, we recommend you check out the answer to What’s the best way to learn if I already speak another Romance language?.\n\nYou are a heritage speaker of the language. You grew up hearing the language around you, but you didn’t speak it yourself. There can be a big range of experiences among heritage speakers. There are differences in the amount of exposure they got to the language, whether the speech was directed to them or not, or in how many different contexts they were exposed to it. Some heritage speakers just heard some of their relatives speak the language to each other, and may be overestimating the total amount of input they actually received. It’s also possible that they only ever heard the same few words and sentences about day-to-day issues, so the input they got was not varied enough. For the most part, heritage speakers that did have at least one parent speak to them in the language most of the time have actually acquired a lot of the language. They may be hesitant about speaking it because they haven’t had the need to, or because they may have had bad experiences with it in the past. Usually these speakers are speaking the language quite fluently after a week or two in a country where the language is spoken. They just needed to realize how much they already knew and lose the fear of speaking it.