Both Starbucks and Safeway are proper nouns, so they should be capitalized.
2. Singular or plural:
Company names are usually collective nouns and are usually treated as singular nouns even when they end in "s", so the verbs and pronouns should also be in the singular form in order to have proper "subject-verb agreement".
3. Apostrophe error:
"Starbucks", "book stores" and "customers" are not being used in the possessive, so they should not have an apostrophe.
As an aside, note the difference between "Starbucks" and "stores": "Starbucks" always has an "s" at the end, while "stores" only has an "s" because it is being used in the plural. As a result, if we were to use "Starbucks" in the possessive, we would add the apostrophe and the "s" as we usually do (e.g. "Starbucks's new CEO"). But if we were to use "stores" in the possessive, we would just the apostrophe and not the extra "s" (e.g. "The book stores' employees").
4. Verb form:
Using "opens" in the simple present tense indicates a habitual action. Instead, we probably want to describe something that began in the past and continues in the present; for that, we should use the present perfect tense ("has opened") or the verb "is" + the predicate adjective "open", or perhaps even the present progressive tense ("is opening").
5. Verb tense:
Try to be consistent with the verb tense. If you use the present perfect ("has opened”) then you should continue using it later in the sentence instead of switching to the past perfect ("had opened").
6. Conditional clauses:
The word "if" introduces a conditional clause. Whenever the main clause uses the simple present tense, the conditional clause should use the simple future tense ("will" + main verb).
7. Word confusion:
Try not to confuse the verb "lose" with the adjective "loose".
A semicolon divides two independent clauses. In this case, the first clause is a dependent one, so we need to use a comma instead of the semicolon.
Try to avoid repeating the same subject/verb construction in a sentence. In this case, we have already used the verb "opened", so we can replace the second instance with something like "in addition to" or "as well as".
Seeing that Starbucks has opened at Safeway stores as well as many bookstores, I wonder if they will now lose customers from their regular outlets.