Welcome and congratulations with your first question 
You have three conditions, Year < 2019, Year == 2019 and Year > 2019, so what you need is a nested ifelse() like so:
# Load libraries
library('tidyverse')
# Since we will be sampling som sales, let's do so reproducible
set.seed(448063)
# Create sales data
d = tibble(Year = seq(1980, 2020),
Sales = runif(41, 10000, 100000))
# Create periods variable using a nested ifelse()
d = d %>%
mutate(Period = ifelse(Year < 2019, "Past",
ifelse(Year == 2019, "Present", "Future")))
Based on you question, I recommend heading over to R4DS to read more
Hope it helps 