It depends. Preschoolers often have problems with pronunciation and difficulties putting sentences together. And that's normal. For instance, many preschoolers still substitute an f or d sound for th ("I'm taking a baf" for "I'm taking a bath") or a w sound for an l or r ("The wion wawed" for "The lion roared"). Consonant blends, where two consonants are right next to each other, are typically tricky ("Soppit!" for "Stop it"). Plus, 3-year-olds often mix up multi-syllabic words or simply reduce them to shorter words ("Gimme dat amal" for "Give me that animal"). All of these mispronunciations are common up until age 6.
Just be sure your preschooler's speech is improving over time — by age 3 most of what your child says should be pretty understandable. However, if your child isn't talking or is talking very little, you should act more quickly. Rule out a hearing problem first. See a speech-language pathologist if your child is doing any of the following:
• Mispronouncing vowels, saying "coo" instead of "cow"
• Talking using mostly vowels, omitting whole consonants, saying "a" for "cat"
• Still saying single words only, and not phrases or sentences
• Using a limited vocabulary. Or saying a word once and then not using it again
• Not pointing to objects in books. If you say, "Show me the kitty cat," he flips the page or repeats the phrase but doesn't actually point to the animal
• Answering a question by repeating part of your question. If you say, "Do you want milk?" he responds by saying, "...you want milk?" instead of nodding his head or saying yes (this is called echolalia, and may be an early sign of autism)
• Not following simple directions and understanding prepositions such as under, on, or over. For instance, he doesn't turn around when you say, "Your ball is right behind you"
• Struggling using pronouns, saying, "Him not here"
• Confusing gender, saying, "He hit me" when he's talking about his sister
• Not changing or developing his language much from month to month
If you think your child may need the help of a speech therapist, talk to his pediatrician, or, if he's in preschool, with his teacher. Chances are his school will refer you to an early intervention program (usually coordinated through the council or school system) that will provide a free screening. Or his doctor can refer you to a private pediatric speech-language pathologist for an evaluation.