It is common for language students to say that they must think in the target language from day one if they wish to succeed. This seems like a sound idea, but can you really "think" in a language of which you know very little vocabulary? I dare to say - even though there are real beginner learners of English in Brazil - that it is practically impossible to find individuals that do not know any English words at all. This being due to the ubiquity of English in Brazilian quotidian communication, from hot dog to software. But then, a few words may not be enough for outlining thought and conveying messages, perhaps only very simple ones. It is then necessary to present learners with a set of basic language items which can in turn set off the thought process and the consequent verbalization of ideas. These basic language items include nouns (pronouns) and verbs. One needs at least a subject and a verb to form a sentence.
OK, so now you have been presented to and practiced a number of words that are now part of your active vocabulary. They will help you put your thoughts together and will be there in speech. But then there is syntax, that, in simple terms, is the order we arrange words in a sentence to convey a message. Each language has its own syntax and a 'hot dog' would be a 'dog hot' in Portuguese. If we assume that thought is conceived through putting words together, it has to happen through syntax. If the learner is not yet familiar with English syntax - though he or she may be using English words in their thoughts - they are still doing so in a frame (syntax) of their first language. A sentence like: 'I like English very much.' might be worked out in the person's mind and come out as 'I like very much English.' which does not follow the expected standard of the English language.
Thought is limited by language. Learning words can only expand your ability to think and to communicate. However, words are not enough.
