I am a native English speaker who has always loved languages. I studied Spanish throughout high school and college and majored in Spanish in college. My husband and I met while I was studying abroad in Spain for a year. My Spanish was quite fluent by this time. He had studied English in high school but had no practice actually speaking it. In fact, he never imagined he would ever need to use his English in "real life." So we spoke exclusively Spanish together, and I quickly became confidently bilingual in both spoken and written Spanish.
We married, and he moved to the United States. He quickly became fluent in English since it was necessary for work. We fell into a pattern of my speaking English and his speaking Spanish in the same conversation. It worked for us (but sounded odd to most other people)! More than 10 years later, we now have two children — ages 4 1/2 and 2.

Raising bilingual children was new territory to us. We were each raised with only one language and had worked diligently as adults to become fluent in our second languages. When our oldest was born, we realized we needed to establish a pattern for the language used at home and decided to speak exclusively Spanish at home. By age 2, our son was able to switch back and forth between English at daycare and Spanish with us.
My fondest success story so far was when we visited family in Spain when he was 2 1/2. We had told his grandmother that he spoke Spanish, but I think she thought he spoke a few words here and there. When he got off the plane and he started speaking to her in complete sentences, she was amazed and said "But he speaks Spanish!" She was overjoyed since they do not speak English. When we returned to the US after three weeks in Spain, he had nearly forgotten his English. He just smiled and laughed at my parents for the first few hours when they spoke English to him. 
But by age 3, with our last trip to Spain six months earlier and weekdays spent at daycare/preschool in English, he began to respond to us only in English, although we continued speaking Spanish to him. We weren't sure what to do. The turning point came when I was chatting with a friend who herself was raised in a bilingual home. Without hesitation, she advised us to force him to speak Spanish. Otherwise, she warned that he would only understand it but never really be confident speaking it. It seemed harsh, but we tried it. We wouldn't respond to his requests unless he spoke Spanish. And to our amazement, it really worked! He would simply rephrase what he said into Spanish. And within a couple weeks, it became natural to him to speak Spanish again. Now, at age
4, we do have to remind him at times to speak only Spanish with us, but we are confident doing that, now that we've seen how well it worked.
We visited Spain again when he was 3 1/2 and were very pleased to see that our son's language skills seemed to be on par with Spanish children his age. To our delight, he was able to communicate easily and naturally with his family and friends there. We could see that he was excited to discover that speaking Spanish is not just a game we play at home; rather, it is vital to communicating with everyone in Spain! Our then 15-month old had about 10 recognizable words at that time, half of which were in Spanish.
On our last trip to Spain (children ages 4 1/2 and 2), we were pleased to see that our youngest easily adapted during our trip. Upon arriving, he danced around his grandparents' living room, chatting and singing to them in Spanish. He spoke four- and five-word sentences and learned a great deal of new vocabulary and expressions during the trip. I think consistency with Spanish at home and the fact that his older sibling speaks to him in Spanish helps a lot.
Upon our return to the US, we have tried to maintain an all-Spanish environment in our home by not only speaking Spanish, but also reading the books we purchased in Spain, viewing Spanish-language children's programs we record from the TV, and switching audio on movies to Spanish. We recently purchased a DVD player that plays DVDs from both the US and Spain, so we can now view children's shows and movies from Spain. (For more tips related to DVD compatibility, view the Bilingual Families Connect Discussion Forum.) Spanish-language Internet resources with games and activities for children are also helpful.
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